I have two GPUs for my early 2008 mac pro (2x 3.2ghz Xeon, 16GB OWC Ram), the first is an EVGA GTX 285 flashed with the mac efi and the second is an Apple radeon 5770. The system runs OS X 10.11.6 just fine.
The strange thing is that in most games on OS X the 5770 outperforms the 285 by a...