Bare Feats compared the NVIDIA GeForce GTX 980 with 10 other GPU's in a 2010 Mac Pro
Bare Feats has a great comparison of the new NVIDIA GTX 980 installed in a MacPro and runs it against 10 other GPU options.
Not only does this card us standard six pin connectors so it will work fine in the MacPro, but in many tests it doubles the performance of a GTX 680 Mac edition! WOW!
If I had the money I would totally go for one of these, since it is literally twice as fast as the GTX 680 Mac Edition, and I currently run a GTX 670, though it does have 4 GB of RAM.
I WANT ONE SO BAD!!! Ha! Not happening anytime soon.
Your best bet for one is this one from MacVidCards which is $835 plus shipping, the cards has been flashed for Mac so you get the full startup with it (which I annoyingly don't have with my GTX 670).