The G4 upgrade is actually pretty easy to do on most systems, I've done many (including slot-loading iMac G3s). Any G3 machine that uses an IBM PPC750L or Motorola XPC/MPC750 CPU can be upgraded to a 7400 or 7410 G4 CPU, as they are pin compatible. The only nuances you have to look out for are CPU VCORE voltage; this needs to be set accordingly before installing your G4 CPU. It is done by changing resistor strap configs, or changing resistor values, depending on the machine. The other thing you need to be aware of is L2 cache I/O voltage (L2OVdd). The 7400 supports 3.3V L2OVdd, however the 7410 only supports 2.5V L2OVdd, and cannot be used on boards that use a 3.3V L2OVdd. Lastly, if you really want to get fancy, you can upgrade the L2 cache. I've upgraded some machines from 512K to 1MB. The following machines can be G4-swapped:
- PowerBook G3 Kanga (maybe, but untested), Wallstreet, PDQ, and Lombard - 7400 only
- PowerBook G3 Pismo - 7410 or 7400
- iBook G3 Clamshell (non-Firewire) - 7410 or 7400
- Some slot-loading iMac G3s (that use PPC750L) - 7410 or 7400
- All tray-loading iMac G3s - 7400 only
- All PowerMac G3s (Beige, B&W) - 7400 only
If you have a machine that can work with a 7410, it is highly recommended. Not only do they run cooler than the 7400, allowing you to increase clock speed a little bit more, but they are much easier to find for sale. I do intend to see if I can modify the L2OVdd voltage on some boards/CPU cards, in order to install a 7410 on all these systems. However, it looks like doing so may be difficult, as these tend to have L2OVdd tied directly to the 3.3V OVdd rail.
I've attached a couple photos below of one of my tray-loading iMac G3 upgrades; with a new 7400 CPU installed, upgraded 1MB of L2 cache, and running Leopard.