Introduction: Part Selection Guide for Building a New Computer

Building a computer is a great way to save some money, or get a better computer for the price.  It allows you to make the computer exactly the way you want it, whether it needs to fit into a certain size, or perform specific tasks.  In this instructable I hope to guide you through the selection process of all the parts needed to build a computer.  For the actual build process, there are plenty of instructables.  I may make one at a later date, but for now, this one is quite good...

As always suggestions on how to improve this instructable are always welcome

Also, here's a random site that I StumbleUpon'ed.  It's pretty out-of-date, but it does have purty animations

Step 1: Decide What the Computer Needs to Do

Put together a wish list of what you want the computer to do.  If you want to run a specific program/game, find the system requirements and consider the 'recommended system specifications' the very minimum you can do.

You also need to decide whether this computer will be one where you build and let it sit at those specs or whether you will upgrade it over time.  I prefer to choose parts that will enable easier/cheaper upgrading down the road.

Decide on a price point.  If you name a price, you can probably build a computer at that price, although it might not be able to do all that you need/want it to do.

Decide on a timeline.  If you have no need of the computer for the next several months, you can sign up for newegg's shell-shocker deals emails and wait for the deals on the parts that you want, as well as watching amazon, tigerdirect, ebay, overstock, and woot!.  Generally the more time you have to wait for deals, the cheaper you can get a computer, although if you take too long you might run into issues with returns (newegg has a 30-60 day return policy for most items, after that your only option is to RMA a defective part for a working one from the manufacturer), etc.

Step 2: Pick a CPU

The CPU is probably the most important part of your build.  The first choice you should make is between AMD and Intel CPUs.  AMD CPUs tend to have lower prices, while Intel tend to have higher performances, and are usually a tad more power efficient.  There are plenty of comparative benchmarks out there, so I would recommend hitting up some hardware review sites like tomshardware or anandtech to see side-by-side benchmark reviews of various computer parts running real-world programs.

One thing I see a lot is people wasting money by choosing the AMD Phenom II x4 965 3.4GHz processor over the x4 955 3.2 GHz processor.  It is incredibly simple to go into BIOS and bump the clock speed up to turn the 955 into a 965, and save yourself ~$10-20

The i3, i5, and i7 all perform very well, in most cases betterthantheirPhenom II counterparts.

Benchmarking websites like the ones listed above are great ways to keep from getting confused by all of the random numbers and specs the manufacturers like to throw at you.  Nevertheless, here is a small description of the key features:

Speed (aka clock, operating frequency):  This is how many Hz (cycles per second) your CPU runs at.  Typically this ranges from 1GHz to 4GHz (1 to 4 billion cycles per second!!!).  Some Intel processors have a function that they call 'Turbo Boost', which basically overclocks the processor on the fly based on current needs (so if you are running a single-threaded application that is really heavily taxing one core, it might, depending on several other factors, heavily overclock that one core and shut down several of the others)

Cache (including possibly L1, L2, and L3):  Processors are really fast.  Really really fast.  They typically run on the order of 100 times faster than the memory.  Since every single bit of every single instruction the computer runs is stored in memory, this means that the CPU needs to read from the memory for every single instruction.  A cache stores the recently used pieces of memory on the CPU, so that it can be accessed much faster (if my memory serves me correctly, L1 is as fast as the CPU, L2 takes ~5 cycles, and L3 takes ~20).  The cache is probably the single biggest factor in the speed of a processor these days.  As a side note, if you're as fascinated by computer architecture as I am, google: cache replacement policy.

TDP (Thermal Design Power, aka Power Consumption):  This is how much power the CPU will draw running at full tilt.  When not in use, the CPU will use a number of tricks (like shutting down unused sections of the CPU)  in order to save power.  Typically this isn't a big deal unless you are designing a computer to go in an area with limited airflow.

Cores (Single- Dual- Triple- Quad- Hexa- and Octa-cores):  In every core, one instruction can be run at a time (false: HT & Pipelining).  Simply put, the more cores you have, the more your CPU can do at once.  Typically with more cores comes better multitasking performance, and better performance with multi-core-optimized programs.

HyperThreading (HT):  This is a technique that some Intel processors use to effectively turn one cores into two.  Basically if two different instructions use different parts of the core, they will both be run at once.

Virtualization Technology (VT):  This is a special set of instructions to help with running virtual machines.  If you don't use a program like VMWare or VirtualBox or Xen, you can ignore this one.  If you do use one of those programs, you'll want a CPU with VT

Step 3: Pick a Motherboard

The motherboard is the next component to choose. 
Several constraints on the motherboard:
1) Pick one that can hold your CPU .  The CPU lists a socket type (LGA**** for intel, AM* for AMD, where the *s are numbers).  The socket type MUST match for your CPU to even fit into the motherboard.
2) Decide on the number of PCI Express slots you need.   Most motherboards come with at least one these days.  If you have a graphics card with PCI-E x16 2.*, you should get a motherboard with a PCI-E x16 2.* port in order to get reasonable speeds (a 2.1 card in a 2.1 slot will also perform slightly better than a 2.1 or 2.0 card in a 2.0 slot)
  - If you intend to use multiple graphics cards, either independently or in SLI/CROSSFIRE mode you will need multiple PCI-E slots
  --If the motherboard has multiple PCI-E slots, it will list the operation modes (x16, x4) (x8, x8).  In general, you will want the x8,x8, or full x16/x16 mode if you plan on using SLI or Crossfire, although these motherboards do tend to cost a bit more.
3) Pick your RAM (next step)
 -Choose a motherboard that has a memory standard which is equal to or higher than what the RAM is listed at
 -DDR2/3 matters.  DDR2 RAM won't fit in a DDR3 slot, and DDR3 won't fit in a DDR2 slot.  DDR3 is far more future-proof
4) Decide on USB 3.0 and SATA III .  Newer (i.e. more expensive) motherboards tend to have these, older ones tend not to.  You need to decide whether you will be using something that will be able to use USB3.0 or SATA3 with this computer over the lifetime of this computer.  If so, it will probably be cheaper and more convenient to get that functionality now rather than skimping out and paying later.

Step 4: Pick the RAM

RAM is probably one of the most confusing parts, despite its seemingly simple job,  due to its large amount of seemingly arbitrary numbers.  Here's some factors to look at for RAM:
DDR/DDR2/DDR3 - DDR2 is faster than DDR1.  There is no appreciable speed difference between two equally-rated DDR2 and DDR3 sticks of RAM, however DDR3 sticks can be rated at far higher speeds than DDR2, and DDR3 tend to be cheaper.  Most new motherboards tend to use DDR3, so you will get the most forward-compatible system by selecting DDR3
DATA RATE - This is the number that comes after the DDR*.  It tends to be something like 800, 1066, 1333, 1600, 1866, or 2133.  The units, for what little they matter, are MegaTransfers per Second (MT/s).  A higher data rate tends to correspond directly with the speed of the computer.  An everyday use machine would do fine with 1333 or 1600, but gaming/video editing rigs could benefit from the higher data rates.  Wikipedia article on DDR3 specs
CAS Latency - This is the amount of cycles between the time when the memory controller tells the memory which address it wants and the time that that data becomes available.  Since it is the amount of CYCLES as a measure of time, the faster the RAM (more cycles per second), the higher the CAS Latency.  In general, pick the speed (data rate) which you want from your RAM, then decide on the CAS Latency.  Switching from DDR3 1333 to DDR3 1600 will probably benefit you more than switching from DDR3 1333 CAS 9 to DDR3 1333 CAS 8 or 7
Heat Spreader - This is basically a heatsink for your RAM.  I would recommend finding RAM with a heat spreader, as it will probably extend the life of your RAM.  There are some situations where you wouldn't want a heat spreader, like if you are doing a build with an ITX motherboard and you put an aftermarket CPU cooler which has a massive fan which overlaps the RAM slots allowing just enough room for the bare RAM.

Step 5: Picking the GPU

There's two different main brands of GPU:  ATI and NVIDIA.  They are both followed by fanboys as ready to fight it out as any mac-PC or Intel-AMD. (ATI is actually a part of AMD now).  If you plan on using any graphics intensive programs, a GPU is recommended.  For a decent gaming GPU, expect to pay at least $100.
GPUs are probably the hardest part to compare apples to apples just by looking at the specs.  Fortunately there are people who have built machines to compare the performance of cards in a series of tests called benchmarks. - My site of choice for benchmarks .  They have reviews of the best value cards any given month, as well as a very active forum community - Also has good benchmark results and forums

Step 6: Pick the Hard Drive

UPDATE: Due to flooding in Thailand, hard drives are really effin expensive.  Your best bet is to either wait until spring/summer to build a PC or use an SSD (or find a used HDD for the interim at some store)

The big thing here is compatibility with the motherboard.  There are two major technologies which are incompatible with each other unless you by a converter:  SATA and PATA (PATA is also referred to simply as ATA or IDE.  SATA is always SATA).
PATA - Parallel Advanced Technology Attachment.  This is the "old-fashioned" style of drive, it has a 40-pin ribbon cable which connects it to the motherboard.  This is far slower than SATA
SATA - Serial Advanced Technology Attachment.  This is the "new" style of drive.  It has a funky L-shaped connector to connect to the motherboard.  The most common right now is SATA II which has a theoretical maximum throughput of 3Gb/s.  SATA III is just emerging, and it has a theoretical maximum throughput of 6Gb/s.  Any SATA II drive should work in a SATA II/III port on the motherboard, and any SATA III drive should work with in a SATA II/III motherboard (full backward/forward compatibility), however if you get a motherboard which only has SATA II it would make no sense to spend the extra money to buy a hard drive that can support SATA III
SIZE - this is the main factor to consider when selecting a hard drive.  Typical sizes are 250GB, 320GB, 500GB, 640GB, 1TB, and 2TB.  Note that these are the metric prefixes where G stands for 10^9 Bytes and T stands for 10^12 Bytes, rather than the way which the operating system measures the drive (G = 2^30 Bytes, T = 2^40 Bytes).  All hard drives use the metric prefixes.
SSD  - An SSD, or Solid-State Drive, is a relatively new technology that is really fast.  Rather than storing data on magnetic platter(s) that spin at 5- to 10-thousand RPM, SSDs use flash.  While a mechanical hard drive has to wait for the needle to position itself over the right track on the platter and wait for the right spot to come underneath it, SSDs simply toggle the right address bits.  Thus, SSDs can acheive a much lower latency, and typically have much higher throughput.  If you want an incredibly fast system get an SSD.  Make sure any SSD you get supports TRIM.
RAID - Redundant Array of Inexpensive Disks.  This is a way to use multiple drives and have them appear as one large drive.  Almost all of the RAID formats need to have drives of equal size and speed (same make and model preferred)
There is a numbering system that is used, for more info visit here
0 - Striping - Data is written across multiple drives, allowing more data to be written at once.  This makes hard disk access faster, however there is no redundancy and if one drive dies, you lose ALL of the data
1 - Mirroring - Data is written identically to multiple drives.  The data on one is an exact replicate (mirror) of the data on the other.  There is no speed advantage, however if one drive dies, no data is lost.
5 - Striping with single redundant drive - Data is striped (RAID 0) across multiple drives, and the last one is used to back up the rest.  The backup drive is the same size as the rest, and stores what is called the 'parity' bit.  If a single drive crashes, all the data is still there and when that drive is replaced the RAID controller will automatically calculate the data which should go there.  If two drives fail, the data is all lost
6 - Striping with double redundant drive - like RAID 5, but with two drives as backups (parity drives).  3 or more drives need to fail before data is lost.
10 (1+0 or 0+1 or 01) - Striping with Mirroring.  Requires 4 drives (or any even number > 4). Data is striped across 2 and those 2 are mirrored to the other 2.  In general I would NOT recommend using RAID as a backup
IMPORTANT: I would NOT recommend using RAID as a backup option for your typical everyday computer.  If a file is deleted, either by accident, program glitch, or virus, the deletion is replicated across all drives.  There is no (easy) way to recover this data.  I would recommend using some other backup solution, like Windows 7's built-in backup (really doesn't like 3TB backup drives), DriveImage XML, Crashplan, or Ghost.

One of my favorite sayings about hard drives is:  "Hard drive failure rate is 100%.  It's not a matter of if, it's a matter of when"

Step 7: The Power Supply

Use a power supply calculator like newegg's to determine what wattage you need for a power supply.  In most cases (pun intended), the standard-sized power supply will work.  Use this list to determine if the power supply you are looking at is one of decent quality or not. 
Maximum Power - This is the single most important factor when choosing a power supply.  Get the maximum power that is higher than what the calculator says you need.  It is generally preferable to include everything you are thinking that you might want to add to the computer, then add about 10-20% as a safety factor. 

ATX12V tends to be the spec you are looking for
High-end GPUs require a separate power connector, either 6-pin or 6+2-pin connectors.  Make sure that your PSU has enough connectors for the GPU.
Also make sure that the PSU has enough SATA connectors for all of the Hard Drives, CD Drives, and anything else which needs a SATA connector.  They do make adapters, so if you become completely enamored with one power supply, you can generally get the connector you need via a cheap (under $5) adapter.

Modular - This is a great way to eliminate cable clutter in the computer case.  The basic connectors are attached to the PSU and the rest are simply cables which you plug in to the PSU if you need them.  On a fully modular PSU, every single cable, including the main motherboard ones, is detachable.

PFC - Power Factor Correction.  This is a complex (pun once again intended) system to correct for the phase angle difference which can be caused by inductive or capacitive loads.  For the most power efficiency, choose Active PFC.

Step 8: Wrapping It All Up: the Case

The case has the simplest function.  They can cost anywhere from $20 to $1000

Form Factor   Make sure that the case is big enough for the motherboard.  ATX should be able to hold ATX, micro ATX (mATX or uATX), and ITX, microATX can hold microATX and ITX, and ITX can only hold ITX

3.5" ('Hard Drive') bays - Make sure you have enough of these for all of your hard drives.  Depending on your GPU you may need to have several extra bays as the GPU can block several (read reviews of the case to see if this is an issue).

5.25" ('CD Drive') bays - Make sure you have enough of these for all of your CD/DVD/Blu-Ray drives (for most uses 2 bays is more than enough)

Fans - Generally shoot for a case that has at least 2 fans, and larger (120 or 140 mm) fans are preferred. 

IMPORTANT:  Small cases might be too small for your graphics card (mainly a problem if you go with a card that is nearly top-of-the-line).  Slim cases won't fit any normal-sized cards (Low-profile cards only).  Take this into consideration.

The Antec 300 and Rosewill CHALLENGER are both great relatively cheap tower cases.

Step 9: Extras

Some things that you also need to look in to getting but are either beyond the scope of this instructable or are simple enough that they really don't need their own step:

Operating System:  If you go with windows, this will cost anywhere from ~$70 to $200, although you might be able to get discounts/free copies through school/work.  Linux is free, Mac is expensive (and limited to specific hardware.  It does work in vmware)
CD/DVD Drives:  ~$20, get one that is also a burner
Blu-Ray Drive:  ~$50
Blu-Ray Burner: ~$80
Keyboard/mouse:  I'd recommend hitting up your local consumer electronics store and finding a set that you are comfortable with.
Monitor:  Once again I think you need to see these in person to get the one you like best.

As always feel free to suggest improvements or your own builds, I will add any that I like/have time to at the bottom..

Step 10: Example Builds:

The build I did for my parents (most of these black friday prices minus rebates, most are cheaper now):
$100: AMD Athlon II x4 640
$80:  MSI 880GM-E43
$45:  Wintec AmpX 2x2GB DDR3 - one stick already failed, awaiting RMA
$80:  MSI R5670 512MB GDDR5 Graphics Card
$25:  APEVIA Case with 500W PSU  -->PSU is very low quality, not recommended
$65x2:  2x 1TB Seagate Barracuda
$20:  CD/DVD drive, forgot what brand, doesn't really matter
The total was about $435 with combo deals and whatnot...