Yes, you can use a mATX moterboard with an ATX case (as far as I know, you should probably check for your specific case). The difference is that its smaller (hence micro) and therefore has less space for RAM slots, VRMs, headers, and all the other goodies. Might even need SO-DIMM RAM (basically, just smaller RAM sticks physically, though that usually mITX boards, which are even smaller).