Well, using an ATX PSU and modding an extension cable is a decent investment because you can just remove the ATX PSUs and stick them in other PCs or modify other extensions for use with other Mac motherboards and such. Even could turn one into a bench power supply if needed.

A $50 Seasonic or Corsair would be able to handle anything those G4s can suck up as well as power a mild modern PC. Seasonic and Corsair are good brands too.