I am thinking of doing the same thing. The Graphite towers are very similar in design to the B&W. AGP graphics, Gigabit ethernet or the best is digital audio (4th PCI slot, 133 mhz bus and AGP 4x) The power supply is different and so is the port layout in back. I would also guess that the motherboard mounts will not match. I really want to do this hack too, If the power supply fits thats half the battle. I think maybe swapping the entire side door would insure the Ports and mounts all line up. Its cheaper to replace the logic board ($100 or so) and then buy the 1 ghz g4 upgrade ($180) than to buy that crappy Sonnet zif for $350.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...