I wouldn’t recommend buying the 8800GT. The card is a bit old and Apple’s support for NVida cards isn’t that great anymore. The official ATI 5770 and 5870 upgrade Kits from the apple store do work in all Mac Pros, even the 2006 models.
Although I went for the cheaper and not officially supported Sapphire ATI 6870 PC card when my 8800gt died a few weeks ago. Costs around 170£. It’s a bit of a hack but works great in my 2008 Mac Pro so far.