I did a project called Ebony and Ivory, just submitted it, where I put a Mac and a PC in one custom built case. Problem with later models is that the case is designed to be an integral part of the cooling system. I tried using a 1 GHz G4 PowerMac, wound up killing the motherboard. Same with an eMac. Finally got an older Gigabit ethernet PowerMac with a 1.4 GHz upgrade board.

You can see the whole build story at http://www.domesticatedarcades.com/ebonyivory2