I don't recommend the G4 upgrades. G3 is just as fast. Some Macintoshes won't accept a CPU upgrade. Otherwise, there may be no way to upgrade the CPU short of replacing the motherboard. Even if there is a motherboard upgrade available, it's usually very, very expensive.
Some people are disappointed with the amount of speed gained from a CPU upgrade.
There are a lot of reasons for disappointment. For example, some applications spend a great deal of time reading and writing to the hard drive. Just because you have a faster CPU doesn't mean the hard drive will work faster. 27 Gig Ultra ATA drive. So, you see, even without a CPU upgrade, it made his machine much zippier.