Using the Old G4 Case

Hey everyone, I've got an old Powermac G4 400mhz and the HDD just went out on me. Now the unit has already been replaced with a Mac Mini and I purchased a Powermac G5, but now am wondering if I am able to use the old case for building a PC sleeper gaming machine. I don't know if the traditional ATX cases geared for AMD & Intel and Apple's are completely different but I fear that they are. If someone wouldn't mind helping me out or letting me know my options, I'd be much obliged! Thanks!