For any user with a MacPro tower, be advised you can purchase PC GPUs that have been flashed for OS X 10.10.x from http://www.MacVidCards.com. I'm running two EVGA GeForce GTX 770ti 2GB cards in my 2008 MacPro 3,1 (using nVidia's own Mac drivers http://www.nvidia.com/download…rResults.aspx/80070/en-us) and am able to leverage the RAM, CUDA cores, OpenCL, and OpenGL capabilities of the cards. No issues at all. The Titan GPUs are also supported on OS X (with the right PSU, of course).
There's no reason to be using an old 512MB GPU in a MacPro! ☺
