It depends on what settings you typically game at.
bash is right that the 2900xt beats the 8800GTS is most cases but from the reviews I have read thats only the case when AA is not enabled. With AA enabled the results flip around and the 8800 beats the 2900 most of the time. Assuming your going to enabled AA I think you would be better off with the 8800.
But I dont have either card so everything I think i know about this is from the reviews Ive read.
You can see in the second page I linked that a single x1950pro gets 13 fs while a single x1900xt or x1950xt gets 17.
I don't have benchmarks for x1950pro in Crossfire but I'm sure they do a bit worse than two x1900xt or two x1950xt in Crossfire.
Based on this, I'd say the single 8800 GTS 320 can beat two x1950pro, or it's very close anyway. I don't know about noise or power consumption but I suspect a single card does better there.
hrmm, i did a favor for my friend, and hell swap his typing pentium D, DDR 2 + pci-e with my P 4 agp cpu...hmm should i option for 2900 pro, 1950 xt/x (299) or the ASUS 7950 (130$), i wonder what HDR (high dynamic range) is?