FYI...
Ok, I bought a MacPro a couple weeks ago. Mistakenly, I didn't look at this forum for graphics card advice and just ordered the stock GT120 from Nvidia. After migration I launched photoshop only to get an open gl error after I opened 8 photos (size and dimension were not relevant, any 9 photos would cause an error) I could still work in Pshop, but I was not able to take advantage of open GL.
So, called apple and told them I wanted to swap out the 120 for the ATI 4870 (after reading the forum and seeing the specs at barefeats) Initially, after a long time being transferred to several folks, Apple said they could do it for 360 or something like that. I told them I would prefer to get the upgrade price as if I ordered it on their web site (I had purchased a BTO from apple.com) The finally obliged with no obligation on my part to send the GT120 back to them. I just got the 4870 today and it is MUCH faster. No errors in Pshop anymore, FCP is faster all is better. Bonus for me is that I kept the GT120 in the mac, in slot 2, and have my 23in cinema display in that card's dual link port, and my 30" cinema display in the 4870's dual link port.
Note: I did't ever run my computer with just the 4870. Tried both first to see if it worked, when it did, I stopped and was happy. So, the speed boost and non open GL error my be due to having BOTH cards in there.. I don't know for certain. I do know that in my previous mac pro I had an ATI x1900 XT and NEVER had any issues there, open GL or otherwise. So, my anecdote says to me, don't get the GT120 unless you you have 2 monitors and the other card is an ATI.