I have put together a small site with a brief tutorial and tools needed as well as the exact hardware I used for a perfect OSX86 machine (two actually this far). I am waiting for more parts to arrive to modify my remaining G4 cases (old G4 Dual 450, G4 1GHz mirror front, G4 Dual 1GHz)

http://www.an43.com/osx/

You'll have some fun fitting components into G4 case hope you measured the motherboard lol. I recently did a G3 case the only mobo I could find that would fit and allow a dvd writer installed was a gigabyte board. It was hell of a tight fit though. You would probably be able to slide a piece of thin card between the back of the writer and the capacitors on the board, but thats about all.