Not sure how to do it with a G5 CPU, but with a G4 you could overclock by moving the transistors around to different positions for different CPU frequencies. You may want to check out xlr8yourmac.com and maybe someone there has already done it.
I strongly suggest that you don't do it. The G5 already runs way too hot in the enclosure as it is and increasing the frequency will only make matters much worse. I understand that you know the risks, but at the same time, its not worth ruining a perfectly good computer just for a few MHz that won't make any difference at all.
But if you want to be geeky and try it, please let us know how it goes and keep a very close eye on CPU and enclosure temps. Good luck!