I'm seeing a frightening number of posts like this, asking questions to compare vastly different cards. Oh well... I primarily blame ATi and nVidia for this mess anyway, by making so many different card models, with a variation level that's hard to tell simply by the model name. The fact that the front-page article falsely compared the GeForce FX as the nVidia equivalent of the Radeon X doesn't help either.
Oh, and yeah, those two cards are in completely seperate leagues; the way I see things, I rank the X850pro as in the 2nd tier, with the FX 5900ultra in the 3rd tier, out of 5 tiers. the X850pro will perhaps get a level of performance approaching some 180% that of the GeForce. the GeForce 6800GS and 6800GT are far more fair comparisons.