How do I know which nvidia graphics cards work with a Mac Pro?
This might not be exactly what your looking for but Bare Feats has several articles on the current state (or lack there of) video cards for the Mac Pro.

Take a look at CUDA compatible GPUs for the Mac Pro and Abandoning Apple 'Blessed' GPUs

I recently had to replace a second dying NVidia 8800 GT in my Mac Pro (early 2008) but decided to stick with an Apple card.  

Most of the non-Apple branded cards will not work until the machine is past the boot volume selection screen (if you need it) and I seem to find myself switching boot volumes frequently.