I’ve installed or recommended nVidia GPU cards for clients and friends for quite awhile now. Personally, I’m rocking a GTX570 in my Mac Pro for Premiere Pro and Resolve. I also have a GT120 installed as the second card for when I use Resolve. Most of the time my monitors are connected to the GTX570. When I jump to Resolve, I switch the monitors to the GT120 so Resolve can use the CUDA processing on the GTX570.
But remember, you need to keep your old graphics card. The drivers that allow your card to work in your Mac Pro right now, will only work with that particular version of the OS. If you’re on Lion, you’ll need a new driver from nVidia when you upgrade to Mountain Lion.
I was reminded of this over the weekend, as I used my Mac Pro to install FCP and Mountain Lion on a new SSD for a client. I booted from my Snow Leopard DVD to install 10.6 on the SSD, so I could then run the 10.8 updater. While booting from the 10.6 DVD, my computer had a kernel panic – the black drape falls over the screen and the multi-language warning comes on screen. Then I remembered, oh yeah, the GTX570 is in there. So I pulled the GTX570 out of the slot (but left it in the computer). With the GT120 remaining in the second slot, and the GTX570 out of the first slot, I restarted the computer from the DVD again and all was well. You may have to do similar things if/when you run your computer from a different OS version than you’re running now.
Along similar lines, if your Mac Pro came with an ATI card that requires power cables, I would suggest buying an nVidia GT120 card as your backup card. If you need to pull your larger nVidia card out of slot one to do an upgrade, it’s easier to pop a single width GT120 in one of the other slots, than pulling the main nVidia card out, pulling the power cables, putting the ATI card back in, attaching the cables; and then reversing the process when you put the nVidia card back in.