Err, if you have Mac Pro and GT 8800, why dont you try it? I've seen recently video on youtube (keywords: nvidia 8800GT mac pro installation) where some guy installes a 8800GT in his mac pro and everthing works. I'm not so sure that it was Mac graphics carde so try it...
Let us know what happened!
Can't try it yet. ;-) Got a Mac Pro on order, with the default GT 120 in there, but the GT 8800 is clearly a better card, which I do have lying around.
The other possibility is to get the ATI 4870 and flash that. Decisions, decisions...
Apple's done well in making the Mac Pro (apart from they could do with a Mac Pro Mini ;-)) a great computer with excellent upgrade options, but I really wish they'd allow for straight stock PCI-e gfx cards to be installed.