hmm, the 6800ULTRA SLI would be about as fast at a 7800GRX (or maybe just a tad faster), and the 7800GTX is about as fast as the 8600GT(S) (in some games the 7800gtx is faster, in some the 8600gts)
so My pick would be the 8600GT, mainly because it uses less power than 2 6800's, and also, you could add a 2nd one. (not to mention you'd only have 1 grafic's card fan, instead of 2, so the noise should be a bit less too )
Dude, the 6800 is ancient crap. I think I would even choose a 7600GT over the 6800 IMO. In other words, the 8600GTS would be the better choice but not the better choice overall. Overall, as Oscar mentioned, get the 8800GS which seems to be the best choice in your budget.