Is this necessarily true? If you can get drivers for the video card, wouldn't it work?
The newest cards need to be flashed to Mac firmware. For example, you can buy pre-flashed GTX 770s like this:
http://www.ebay.com/itm/Nvidia-GTX-770-2-GB-for-Apple-Mac-Pr...
http://wiki.osx86project.org/wiki/index.php/HCL_10.8.4#Graph...