Well, given that for normal use, the stock graphics card in a 2008 Mac Pro is just as fast, there's not much reason to move the card. The 8800GT uses the same GPUs as the current Quadro FX, and for normal use there's no difference.
As it is, the ATI 3870 (or whatever the number is) is the best overall graphics card for Mac Pros, and it's compatible with both models.