Overclocking: How much do you want to know?
If you want to get past the practical "just make it go faster" side of things and look into what makes overclocking work on a lower level, check out this article. It discusses everything from why to overclock (with a nifty graph showing how prices shoot up as clock speed increases) to how it works, including such niceties as detailed descriptions of how the clock signal in a processor works and what it does. I'll warn you now; this article is pretty hardcore. If you're really interested in the internal workings of a processor and what determines the max. overclock for a particular chip, it's definitely worth reading.