It should probably be pointed out that G4 cases are very difficult to modify for regular atx motherboards and components. The motherboard attaches to the wrong side of the case, which folds down when opened. This can be overcome, and there are guides available with some googling, but this doesn't seem like what you want to do (and boy does it look like a pain). the G4 cases also didn't have super airflow, making it harder to keep them quiet.
G5/Mac Pro cases are easier to modify but are more expensive and there would still be plenty of metal work involved.
I've never seen a 'mac-like' case that wasn't hideous, so my suggestion is to give up on the mac factor and build a myth box that looks like a myth box. there are plenty of cases around that aren't ungodly hideous, and you could always go for the 'media center' form factor as well.