The Extensa 5620-6830 comes with 1GB of DDR2 memory. That's two sticks of 512MB. This unit technically supports 4GB; 2 x 2GB modules. However, unless you're running a 64-bit operating system (if you have to ask how you'd know, trust me... you're not) then only about 3GB will be usable. There's no fix for this, it's just a fact. All standard 32-bit operating systems are coded this way.
Due to this, I decided to purchase two 1GB modules (making 2GB total). I find that to be plenty sufficient; I've never run out of memory in XP. Keep in mind, Vista will use about 1GB just to boot and run. You'll want to buy 667Mhz modules. There are 800MHz modules out there, but 99% of laptops on the market can't operate at that frequency. You'd be paying extra money for nothing, plus risking incompatibility.
There is an obvious single "hatch" on the bottom of the unit through which we access nearly everything- wifi card, RAM, hard drive, and CPU. You'll need a small philips-head screwdriver to open this. You shouldn't have to apply force once the screws are out, though take note of the plastic tabs along one side.
The stock hard drive is a TOSHIBA MK2035GSS 200GB drive. It runs at 4200RPM, has an 8MB Cache, and uses Perpendicular Magnetic Recording (PMR) combined with Tunnel Magneto-resistive Recording (TMR) Head Technology. All that jargon supposedly means it's roughly as fast as a 5400rpm drive. I find it acceptable. There's faster drives out there, but is it worth wasting the $165 on a 200GB 7200rpm drive
just to replace the original 200GB? You'd see a lot more heat, less battery life, and a big chunk out of your pocketbook.
The choice is yours, but I'm waiting for a faster (or equivelent-speed) 320GB drive.