It can be done, but it probably won't be nice. Mac motherboards are made to fit in Mac cases not standard PC cases. The main problems are the ports won't match the port area of the case, and the screws that hold the motherboard to the case won't match up to the screw posts/holes in the case. The power button and any other front buttons probably won't match.
There have been a few of these "conversions" done and there are pages on the 'net about the conversions.