On a windows box you also open up the options for Houdini, max, mudbox, and Z3 (although the mac versions in the pipeline..... but think its had a massive blockage though) as well as many plug ins for maya.

I would also have PS on the windows box, just then you dont have to bother going between one system and the other when texturing.

As for motherobard etc, you need to figure out what processor you want.