Here's some info on replacing fans in the G4 Quicksilver. What's cool is the guy pulled out the front speaker (now mounted elsewhere) and replaced it with a LCD temperature monitor (backlit) connected to the CPU. He arranged the monitor so the temperature shows the speaker hole.

I'm sure this mod could be modified to other uses and other G4s.

http://g4noise.com/mods.php?ID=77&p=all