Your 7900GS wont run crysis at high :p, have you seen the screenshots??? Even my rig will be pushing it trying to run it on high and I have an 8800GTS, my CPU will be letitng me down, and your CPU will to, even your CPU would be bottlenecking a card like an 8800series.
Whats a 6800GTX? do you mean GT? I dont think Doom3 gave that a run for its money, I had an X800XTPE, slightly more powerfull than a 6800Ultra, and I could max doom 3 out no problem, the graphics aint that good on Doom3 compared to modern games :p
Its unknown weather the R600 will be the best GPU by far, at this point we dont know and it may not be that much faster than the 8800GTX, allthough if it aint faster, AMD really need to get there act together.