Here are the 8800 series line:
8800 Ultra
8800 gtx
8800 gts 640mb
8800 gts 320mb
A lot of people say the ultra is an oc edition from the gtx and it has better cooling.
And the 8800 series can play dx 10 games...the X1950 pro cannot.
If you want future gaming proof, id say get the gtx or ultra if your not on the budget..
Im on the urge of returning my gts and getting the gtx.