Have a look on this site:

http://power-mac-g4.com/g4clockup.html

I only glanced at it but it might have the jumper settings you need.

I have heard that certain G4s are prone to instability when the bus is OC'ed.
Just remember if you do it, it will push the CPU speed up even further, so if you have the CPU OS'ed already, you could melt it if you don't clock back down to do the bus.