I like the X1900XT/X and would go with that. The reason is because I like HDR + AA, I've seen HQ AF in BF2 and looks awsome, Adaptive AA, and they are in stock and cheaper as well. However, the X1900XT/X uses more power and is more hot but isn't a big factor. They also said it is really loud, but when I heard my friends computer you could tell it was louder then other video cards but when your playing a game you don't notice it as much and if you have headphones your fine. ATI has better image quality the nvidia and it handles AA/AF pretty good while Nvidia's cards run slightly faster most of the time, but I'll take HQAF over 6 fps. In 3Dmark05 the X1900XTX beats out the 7900GTX, but in 3Dmark06 the 7900GTX beats it out by ~150 marks I think.
I even hear that the X1900 runs better in Oblivion in outdoor areas then the 7900GTX but perform almost the exact same in indoor areas (even with the so called Oblivion optimized drivers which hasn't shown much of an improvment).
Both are great cards but to break it down, I like better image quality and cheaper price over 5-10% performance increase in ideal conditions (where as the X1900XT/X is either slightly slower or faster).
In stock and cheaper ($4xx range)
Best image quality (HDR+AA, HQAF, AAA)
Louder then most cards
Uses more power then 7900GTX
Runs hotter as well
Crossfire isn't that great (clunky setup, not as efficient as nvidia, uses more power)
Quiet and decently low power use
Slightly better benchmarks (single card)
SLI owns Crossfire
Not a lot are in stock