It's going to be almost neck and neck in performance, however, you'd be quite mad to buy a pair of 7900GTX's at their current price.
When the 8800GTX launched it was worthwhile going 7900 SLi, but with the prices what they are now, it's really not. Unless you can pick them up quite cheap, it would save a lot of time and hassle if you just got the GTX.
http://www.anandtech.com/video/showdoc.aspx?i=2870
That Anandtech review above compares the 8800 cards to a variety of last-gen setups, including the X1950XTX CF and 7900GTX SLi. As you can see, the performance is near neck and neck. However, the IQ is a LOT better, and a single 8800 doesn't need you to fiddle with SLi profiles either.
Unless the prices change a lot, i'm going to say it's 8800GTX all-the-way.
[Edit] Also note that the BFG 8800GTX price appears to be a "this week only" offer!