I've been doing some research (which means Google and surfing the web) on the options for upgrading the video card in the Mac Pro. So far I've only heard of limited success with off the shelf cards. Here's a quick summary.
x1900 xtx has been reported by ONE person GeForce 7900GTX only works under Windows with BootCamp PC version of x1900 xt is claimed all over not to work on the Mac Pro SLI on the Mac Pro will only work with BootCamp
So my is guess that we're looking at a few months until Nvidia starts announcing official Mac Pro/EFI compatible cards. Now that ATI is part of AMD, they may never release a new card, except for a couple editions of the 19XX series. My biggest hope is that Nvidia jumps on this market and figures out a solution for the SLI issue. Don't get me wrong. The Nvidia GeForce 7300 GT that came stock in the Mac Pro is quite the impressive video card for the Mac market (Call of Duty 2 is liquid smooth). But I build my own PC boxes and I love to push the hardware limits. It's really a double edged sword though. The more options you push on the OS (needing driver support) the less robust the system could be. I will take stability over speed in my workhorse machine, thank you very much.
Here's a great place to start if you have more questions about upgrading the Mac Pro