Performance atm is the same. The X1900 has better IQ but runs far hotter (and draws much more power).The 7900 is a much more power-efficient design - and a much smaller core.
Depending on how much pixel shader and SM3.0 stuff new games will use, the X1900 will pull ahead - its a more powerful chip in reality, but games dont use that power atm.
If i was planning to run 2 cards, id go for the 7900. The extra power draw, heat and the maturity of SLi (both in hardware and software) vs crossfire makes it a winner imo. Ofc, price is an issue too
Also, the stock cooler on the x1900 is criminally poor - were talking 90c (on XTXs) @ stock (and sometimes crashes) easily in well-vented cases. So if you do get one, do yourself (and your ears) a favour and get a VF900. The Accelero X2 wont improve temps (though it is quieter).
As far as im concerned - I heart my X1900XTX, but im cooling it with H20
Oh, and I only ever run Omega drivers on my ATI cards