Heh, I know I'm probably in the minority around here, but the old plastic G4 cases look like cheap garbage to me. I will say, the sawtooth cases look a lot better than the old G3 garish blue cases- good lord what an eyesore those were. The steel G5 cases are the beginning of truly great/modern case design from Apple, but before that- bleh. "The circus is in town" summed up the age of bondi blue, garish multicolored 'powerpuff' ibooks (where's the puke icon?) bulky ugly iMacs, plastic tub PowerMacs, etc...
Rant aside- interesting project. I've seen many people do the opposite- old G4 case, new guts, but never this- old G4 guts, new case. You certainly buck the trends!
I have to wonder though, is it really worth it to put a dime more money and time into such older hardware? Throw a new hackintosh board in a new case that'll run rings around that, and retire the old G4. (Granted, boring in comparison.)
Anyway, just my 1.5 cents. Interested to see how this progresses.