Overclocking chips is possible, though not recommended. There are plenty of sites on the web that will instruct on how to do this, so I would suggest Googling for them. Each site will state different possiblities on speed increases.
Keep in mind, that as previously stated, overclocking significantly decreases the life of your processor. The increased heat is going to damage the chip, even if you manage to up the cooling with more fans, etc. The damage will be gradual, and you may not notice it at first, but it will damage the chip over time.
You also run the chance of damaging the logic board if you are unfamiliar with overclocking, even with a detailed set of instructions.
The biggest risk however, is that you void any and all warranty, APP included, if you alter the chip by overclocking. Even if something were to go wrong, and you attempt to underclock the chip back to factory spec, it will be detectable that it was overclocked.
Some PC chips are underclocked at the factory, or at least have more "room to grow" as it were, so overclocking is not as much of a risk with them (AMD in particular), but the risk is there nonetheless.
So, in my opinion, it would be in your best interests to just forget about overclocking your Mac. It really isn't worth the minor speed increase you may acheive from doing so.
