I would Say its the Answer to the 6800NU, both being 12 pipes.
The 6600GT's now can be found regularly for $160(but are only 8 pipes), and 6800 NU's for $199.
Its very comparible to the 6800NU. The 800 seems to be a bit faster in most tests probably becuase the 800 uses ddr3, and the 6800's do not.
But the 6800 has been cheaper and more available for quite awhile. I have a 6800 myself. But if the prices are the same, the 800 probably is a slighty better buy becuase of the ddr3.
Just my 2 cents.
Edit: after further review... anyone have and answer to this question.. Why does the 6600GT have ddr3 mem clocked at 1000, but the 800's are clocked at 700? I know there must be a reason, but it escapes me, and i have less experience with ATI then I do Nvidia.