Well Sonnet sells anything up to a 1.4GHz G4 CPU upgrade for the PowerMac G4 AGP. I imagine PowerLogix would have similar upgrades. The higher the MHz, the costlier the upgrade will be. Giga Designs also makes CPU upgrades and they are supposed to be cheaper http://www.gigadesigns.com/products.html
Personally, I've never upgraded the CPU in a machine. By the time I'm thinking of upgrading, it usually makes more sense to buy a new machine and get the advantages of the new technology and bus speed.
Yeah, but does it make any sense spending that cash on a dual G4 upgrade for a machine with a 100Mhz bus and a 2x AGP (actually it's probably just a 1x AGP). If your considering a dual upgrade, you're probably best to put that cash towards a new G5 or G4 with a faster bus, 4x or 8x AGP and all the other new technologies that come with the new G5 or G4.
I see the 1 + GHz upgrades useful for machines that have a 133 MHz bus/4X-AGP graphics, but for anything less, I wouldn't recommend getting an upgrade card beyond 800 MHz G4 w/1 MB (or 2 MB) L3 cache - which is still a pretty fast processor.