You need Apple drivers, and Apple only makes drivers for video cards they support. So it would not be quite that straightforward.
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...