Check out these guys:

They make software tools to get around the 128gb limit, Though I have never personally used it. I believe people have had mixed results with their products so do some research before you commit. But it at least opens the possibility of a larger drive.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...