First of all, I would like to mention that OCing of Intel CPUs can be done in two main fundamental ways:
1) Increasing the Front Side Bus (FSB), which means to increase the data transfer rate between the CPU and RAM
2) Increasing the multiplier, this option isn't available to most users since almost all Intel CPUs apart for the Pentium Extreme Edition and Core 2 Extreme have their multipliers locked.
Another thing is that we would be using the BIOS to perform all our tweaks, there are various softwares out there, but they only support a limited number of motherboards and are not that reliable. Also note that when you OC CPUs, you are also OCing your motherboard (in which the FSB is implanted) and your RAM. OCing RAM is notoriously tough and risky, but you can deal with this issue by decreasing the FSB:DRAM ratio we will be discussing later.
Ideally, you'd want these motherboards to OC on:
These motherboards are not so good at OCing:
Intel 946 chipset (though I personally have it and managed a 25% OC without any problems)
Perhaps the biggest problem with OCing any component is overheating. If you want really high OCing capacity of 70%+ then nothing other then a water or liquid cooling system will do. mid-range to high-end air cooling is known to be able to OC to around 60% while stock cooling can only reach 30% at max (my E2160 has stock cooling and a 25% OC). There is no exact temperature limit for any CPU. But as a general rule, you wouldn't want your temperature at any more then 75 (degrees celsius) under load. Having it at above 60 degrees shortens the lifespan but that shouldn't be a worry if you don't plan on using your CPU for more then 2 years.Recommended software
To check for basic statistics regarding your CPU, I strongly suggest that you download the CPU-Z utility from:http://www.filehippo.com/download_cpuz/
And get Speedfan for closely monitoring your temperatures, but before this, look around the CDs your motherboard came with for a specific utility for monitoring them. ASUS motherboards for example, come with ASUS PC-Probe which is far more accurate then universal monitors. But here's the download link if you don't have one:http://www.filehippo.com/download_speedfan/
Also, download a simple application called SuperPI, which essentially gives your CPU a task to do and times how long it takes for the task's completion. Note that this application doesn't support multi-threading meaning it can only utilize the power of one core even if you have 2 or 4 But the purpose here is to check for basic stability and performance increase. You can get it form here:http://www.techpowerup.com/downloads/366/Super_PI_Mod_v1.5.html