Officially, the PC variants of the Nvidia cards will not work in any of the mac pros under OS X. The mac versions of the nvidia cards have mac specific boot instructions for OS X operation. You could add a PC variant of a nvidia video card to a mac pro, but it would only be useable by default under boot camp/windows. I believe the GTX 285 mac version was the only 200 series card produced for macs, so that may be your only option.
Unofficially, there are people that have attempted to flash similar Nvidia cards for the mac platform and have gotten specific models to work to some degree. This is obviously not supported by apple. I am just saying it is technically possible with some specific cards. You need specific models with specific sizes of memory. This comes with the risk of bricking video cards (incorrect flash, errors, etc.), boot issues, and various other inconsistencies with using unsupported hardware in apple machines. You can google for various mac/pc video card flashing information sites around the web that discuss user success and failure experiences.