If you have ever tried to find a third party video card for a Mac Pro, you know that options are few and far between. NVIDIA and Radeon have limited the selection of video cards marketed to the Apple community for quite some time; there are few users in the Mac community interested in upgrading the GPU in their Mac Pros because, well, there are just so few people who purchase and use Mac Pros.
This has led to a market place that is minimal at best. If you want to replace the video card in your out-of-warranty Mac Pro, often the only solution is to purchase an OEM replacement from a service provider (like Small Dog), or purchase used OEM cards online.
NVIDIA offers drivers on their website for many of the aftermarket graphics cards for Mac clientele. With a little quick digging, you can find new driver releases for Quadro and GeForce cards that will work with their aftermarket cards. For example, NVIDIA drive 270.00.00 supports, in Lion 10.7.3, a few cards that are not Mac specific (February 2012 release).
http://www.nvidia.com/object/macosx-270.00.00f01-driver.html
The driver set is an update that works on Mac Pro 2008, 2009 and 2010. The cards supported by this driver release include the off-the-shelf GeForce 8800 series.
The only problem with this process is that you need to be able to see the information on screen to install the driver. In cases of a failed video card, you’ll need to access the machine remotely or install a known-good OEM card to complete the process.