After I looked at some 6800 XT benchmarks and game performance, I can to a conclusion that the 6800 XT performs about the same or a little less then the 6800 (vanilla). I could not find any 6800 XT SLI reviews so I decided to find the next best thing, 6800 SLI reviews.
I like firingsquad's reviews. After looking through I found that the 6800 GT performs about the same a SLI 6800s. However, these are on older video drivers, and I'm not sure how they perform on the latest drivers. The 7600 GT performs a lot like the 6800 GT.
I would go with the 7600 GT for a couple of reasons:
1) The GeForce 7 series has support for transparent AA (this allows things behind transparent objects like windows and chainlink fences to be rendered in AA as well)
2) Lower noise, heat, and power usage (because only one card is running)
3) Leaves an open slot for another 7600 GT
4) Some games don't run SLI very well
5) The 7600 GT (you picked) comes with GRAW
6) SLI isn't worth buying unless you already have one of the cards IMO