Concerning the X1900 vs 8800GT. I have a good perspective on this.
At home i have a 3GHz first generation Mac Pro with X1900.
At work i have a 2.8Ghz 2nd generation Mac Pro with 8800GT.
Both machines are running 30" monitors and I play a lot of World of Warcraft as well as use pro applications (Aperture and etc).
Both machines run WoW flawless but the 8800GT does offer about a 10-30% gain in FPS depending on the scene you're in.
What really surprised me is that the X1900 runs OSX UI interface MUCH smoother than the 8800GT. Dragging windows is smoother, and when the UI does transition and etc, the X1900 does it smoother. I also notice that the X1900 can do more than one graphic intensive thing at a time without stuttering, unlike the 8800GT.
Here is an example..
If you run your mouse over the Dock and select the Dashboard, if the dock is not done animating (magnifying), the Dashboard will stutter terribly or skip the transition all together on the 8800GT. If you have any other GPU activity going on it'll do the same thing. On the X1900 it's ALWAYS smooth. I notice this sort of behavior in several places.
Even though i play WoW, i would buy the X1900 over the 8800GT because of how much better the X1900 runs OSX in general. I will be one of the first in line to get the ATI 3870