Bah, ignore this lot 
Get the Mac Pro with the stock GT120. You won't be gaming under OS X, and under Windows you can run any card you want basically. Simply go and buy an off-the-shelf PC card of whatever kind you like and use that under Windows.
Case in point, my Mac Pro has a 2600XT and a GT120 for OS X use, and when I boot into Windows I use a GTX 275 to drive the 30" display for games. Works great, and cost a fraction of the EVGA 285 for Mac.
Get the Mac Pro with the stock GT120. You won't be gaming under OS X, and under Windows you can run any card you want basically. Simply go and buy an off-the-shelf PC card of whatever kind you like and use that under Windows.
Case in point, my Mac Pro has a 2600XT and a GT120 for OS X use, and when I boot into Windows I use a GTX 275 to drive the 30" display for games. Works great, and cost a fraction of the EVGA 285 for Mac.