I’m running two 7900GT’s so I don’t know if you would see the same kind of performance increase, but I saw a 2000+ increase in 3D ’05 & ’06 and on the F.E.A.R settings test @1024x768 with all the other settings maxed out I get 76fps with a single card and in Sli mode I get 114 fps, if those are the sort of increases that you are expecting and are willing to pay your $350CA for then the rest is up to you.
It's not really about the Dx10 bandwagon though, is it? Dx10 has little to do with it.
It's about the best performance for the dollar in today's games.
I think the 8800 would probably give you that more than two 7900 GTOs.
Dx10 would be just a bonus, in addition to that SLI presents it's own challenges and requirements. Plus, with the next gen cards coming out now, the 7900 GTOs will drop in price pretty quick, and a single 8800 would give you upgrade space to SLI in the future...
Ah well, it's your money, but if I were you I'd take a long hard look at the benches.