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.