Arcticsid asked the Answer Line forum to explain overclocking, and why it is, or is not, a good idea.
When you overclock a hardware component–usually the processor–you trick it into working faster than the manufacturer’s recommended maximum speed. Every processor is packaged and priced to run at a particular clock speed–for instance, 3100 MHz. But that speed is an estimate, and usually a conservative one. You can usually bump it up a bit without causing problems. Sometimes, you can bump it up quite a bit.
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You may also be able to overclock other components, such as RAM or your graphics processor. But the term generally applies to the main processor, and that’s what I’ll be discussing here.
Why do it? Overclocking your processor speeds up your computer. That’s a pretty good reason.
But there are some equally good reasons not to. For one, it might damage your hardware, although bringing up the speed a notch or two is probably safe.
Keeping your PC cool will help protect it from damage caused by overclocking. So make sure your PC is well-ventilated before increasing its speed. I don’t recommend overclocking laptops, which tend to be less ventilated than desktops.
Even if overclocking doesn’t do any harm, it could still void your warranty. So if you’re thinking of overclocking a PC new enough to still have a warranty, check with the manufacturer first.
Finally, overclocking may not improve overall performance all that much. The processor is only one of several components that effect overall performance, and it’s likely not the one that’s slowing you down. Hard drives, RAM, and graphics may be more serious bottlenecks.

You can generally overclock your processor through your BIOS Setup utility. However, I strongly suggest you do some research on your processor and BIOS first, so that you understand what you’re doing.
Read the original forum discussion.