**UPDATE** I accidentally put GTX 600 before but I meant to say GTX 660. since I didnt see a bump button here I am re-posting.
So I got an Mac Pro 1.1 which I upgraded to Mac Pro 2.1 running Yosemite (Pike). I see people had some luck with PC versions of GTX 660. So I thought I would get one on ebay. However I noticed there was different flavors of this GFX card and I am unsure if it matters or not. There is GTX 660ti, GTX 660SC, and GTX 660FTW does anyone know if any of these will work in my Mac Pro? I do understand I wont see the boot loader screen. Any help would be great. thank you.
Sorry to reply to an old post - Perhaps you've already made a decision on the GPU; but, in case you haven't yet decided: I got a GTX 660 OC for my Mac Pro, and it works just fine (albeit a better performer in Windows, due to the fact that the card isn't flashed). In fact, the GTX 660 is a recommended card for these things. That being said, I heard (but can't quite confirm) that the 'Ti' version of the GTX 660 has issues. I'm not sure what's true, but I decided to avoid it just in case. I ended up going with my 'OC' (which is, I believe, the same as an 'SC'), which has the better, ASUS CUII cooler on it. Note that you need a [single] power cable for your card.
Of course, if you want a more beefy card, I hear that the GTX 960 also works fine - just like the GTX 660 does. You can get one now for around $200. Pretty sure it, too, uses a single power cable to run.
Anyway, my GTX 660 is still going strong. Had it in my 1,1 and then transferred it to my later 4,1 Mac Pro - no issues. It's a pretty good performer for its age. Overall, I did notice that, as a non-flashed card, 'Stacks' in Mac OSX El Cap - WITH transparency enabled - is slightly laggy, but all other animations are fine. I like transparency off, though, because it induces unnecessary graphical stress. As for in native Windows, it seems to do very well with the games I play. I'm able to max out almost every single option in Grand Theft Auto V, and I get acceptable frame-rates.