As long as you are running OS X 10.8.2, GTX series cards will work in the Mac. There are no Mac versions of GTX cards. Also note that all this is related to Resolve specifically. Other apps may not like GTX cards as well or at all.
If the GTX card is the only card you have, it will not show you the Mac boot screen. Most people don't seem to care. But you should be aware. Once OS X is running, GTX cards work normally.
You should visit the NVIDIA Driver Download site and download the latest GPU driver for OS X 10.8.2. This is most easily found by searching for: Quadro /Quadro Series / Quadro 4000 for Mac / Quadro ODE Graphics Driver. Today's driver is :304.00.05f02 - Link:
http://www.nvidia.com/object/macosx-304 ... river.htmlYou should use CUDA 5.0.37.
Since your 2008 Model 3,1 MacPro is Resolve performance limited, you should not get the best card, as it won't help performance. A GTX570 card would be fine.
If you are only going to use a single card, and you want total Apple compatibility and a boot screen, the Quadro 4000 for Mac card would work fine. It's performance in Resolve is not as good as a GTX570, and it costs more, but if you want a boot screen and perhaps better compatibility with other apps, it might be a way to go. And the lower performance won't matter in a 3,1 MacPro.
GTX cards normally come in 2 versions, one with more VRAM than the other. We recommend that you get the card with the most VRAM. The Quadro 4000 for Mac is only available with 2GB of VRAM. If you get a GTX card, we would recommend it have at least that much VRAM.
Your lowest cost option would probably be an ATI HD5770 for Mac card. This will use OpenCL. This will only work with OS X 10.8.2. This will not support Resolve Noise Reduction. But in a 3,1 MacPro, I'm not sure the performance will be much worse than an NVIDIA card. It probably will be somewhat worse. If you don't care about NR, and you want cheap, this could be a choice.