Mounting NuBus motherboards in ATX (or older AT) style cases is problematical because of the expansion slots. The NuBus slot openings have those little nubs for the NuBus card backplane thingys to hook onto and the ATX has nothing similar.
To really make it work, you'd probably need to do some custom sheet metal for the backplane of the case.
But in general, and if you don't care too much about the card fit, yeah, it can be done. You'll need to look up the pinout for the ATX power supply and compare it to the Q950 PS to get the pinouts correct. Also, you'll probably need to put an inverter on the Power-on wire, because I think the Mac expects 5V and the ATX expect GND.
To really make it work, you'd probably need to do some custom sheet metal for the backplane of the case.
But in general, and if you don't care too much about the card fit, yeah, it can be done. You'll need to look up the pinout for the ATX power supply and compare it to the Q950 PS to get the pinouts correct. Also, you'll probably need to put an inverter on the Power-on wire, because I think the Mac expects 5V and the ATX expect GND.