Bunsen said:

You can also use a ZIF G4 without issue (once you've set the motherboard jumpers to the correct speed, same as with a G3 upgrade)

Click to expand...

One caution there; I seem to recall there was some sort of issue with the VRM modules shipped in a few Beige G3s that made them likely to fail when used with G4 CPUs and said failure stood a chance of destroying the CPU along with the VRM... (*googlegoogle*) AHA:

Here's the info.

Edit: This link says it could be a problem with faster G3 upgrades as well.