I hade a friend of mine who's at WWDC ask an Apple engineer about the "Note: The Mac Pro does not simultaneously support an ATI graphics card and an Nvidia graphics card." quote from
http://developer.apple.com/document...le_ref/doc/uid/TP40003994-DontLinkElementID_1.
Apparently, that quote only means that you can't run an ATI Radeon X1900 XT and Nvidia Quadro 4500 FX simultaneously. However, they tried putting a Geforce 7300 GT in with the Radeon card, and that worked fine! So the quote is quite misleading, and my setup should work beautifully.
The Apple engineer also mentioned that you no longer need special mac firmware on the graphics cards, so you can use any generic PCI Express card! This has not been verified though, but should give you some hope at least
🙂