xlr8yourmac is a great site. You would have a hard time overclocking an imac, because Apple doesn't make it easy to do. I believe that you'd have the same issues trying to overclock a Dell or Gateway PC. You have more control when you buy components and build a PC yourself, which isn't really an option with a Mac.
However, I've overclocked my PowerComputing mac clone (kinda old) with a G3 upgrade card. The companies that make upgrades (xlr8, newertech, sonnet) often include dip switches on the card, much the same as you'd find on a PC motherboard. You can use these to overclock your processor and bus. I'm sure that you can find these for imacs nowadays. My 300Mhz G3 worked reliably at 333Mhz, and I got the 60Mhz PowerComputing bus up to 64Mhz.
Many Anandtechers will scoff at this point. This upgrade was done a while ago, and the machine is more than 4 years old. I switched to PCs for the price/performance ratio, and control I gain by building my own machine. But my Mac still has a place as a server, and it works great in it's current arrangement.
I'd recommend Other World Computing (not sure of web address) as a good source for upgrade cards.