If the 7800GTX is too much, than go fot the 7800GT. it has 20 pipelines and 1 less vertex unit than the GTX, but still has all the anti-aliasing bells and whistles.
The most recent edition of CPU had an article about the GT 7800 that claimed that a single 7800 outperformed 2 6800s in SLI. That's worth checking out. I'm getting 2 of the 7800s for an upcoming build. I have yet to see hard core specs from Benchmarks though.