Yeah, I'd suggest ebay at this point unless you're married to that G4 for the next 2-3 years. If that's the case, you could buy from OWC; I bought a dual 1.3gHz for my Sawtooth a couple years back and, since at the time it was my main machine, it was DEFINITELY worth it. The RAM ceiling became a factor when I started using CS3, though, which is why I switched to a G5. I still use the G4 for recording and mixing audio, though, works great, and I'm glad I still have the upgrade...
So yeah. Not really worth the money unless you plan to use the system regularly for a while. They also generate a LOT more heat than the stock processors, so be ready for your G4 to double as a space heater! Replacing them is super easy, you just pop off the heat sink brackets, remove it, unscrew three screws, and pull up. Be careful when putting in the new card as the connectors are a bit sensitive; when popping it in you'll know you're good if the screw mounts are lined up with the screw holes. It's not hard at all (and most of the upgrades come with heatsinks already mounted, so no worries there).