Hey truffle,
7900 SLI are faster, ATI gives you the HDR+AA feature.
However, under no circumstances, even if I would be doing $50K monhtly, I would buy any graphics card.
Wait for DX10 and the first DX10 cards. It's pretty safe to assume that the first DX10 card from either nVidia or ATI will be faster than X1900 XTX and faster than 7900 SLI (or maybe at 80-90% of their speed).
That's my POV, truffle.
If you want to play HL2, F.E.A.R. + XP, C&C3, Quake IV, GRAW: PC, Oblivion etc. - any current and near future PC title INCLUDING Crysis in 1024x768, no AA, DX9 mode in full speed, ultra-smooth then go for it and pick ATI X1900 XTX. DO NOT BUY 7900 SLIs, they are superfast however they draw more power give more heat and are noiser. Besides you'll be stuck with 2 cards to sell once the 2nd gen of DX10 cards roll out.
There are rumours that DX10 cards will feature multicore GPUs.
So if you wanna play it smart wait for Vista and DX10 cards. In the meantime tug along with what you've got and enjoy your Xbox 360.
If you wanna enjoy the newest PC games NOW then buy X1900 XTX now. You'll be kinda sorry when Crysis rolls out and you'll read about the difference DX10 hardware makes.
Make no mistake - X1900 XTX is a beast but DX10 titles are looming on the horizon.
Personaly, I'd wait, hell you've got a Xbox 360, you can game almost all PC DX9 titles there.