Most processor families do not overclock very well, including early generations of PPC. The later G3 is clockable -- 20% in terms of MHz if you are prepared to add extra cooling. But whatever your processor, get all of the "cheap" speed elsewhere before you overclock.

"Cheap" speed: Have you provided enough RAM for the apps that you wish to run? If you have enough physical RAM, did you disable Virtual Memory? What is the fastest hard disk option? On-board SCSI or PCI ATA? Will Speed Doubler accelerate any 68K legacy code? Surely the fast sensible (2D) video card?