Seriously, there is no need to buy a $500 card. You are better of saving $150-$200 buy buying a midrange card that will still last you 1 - 2 years.
But for the X800 XT vs. GeForce 6800 Ultra, when it really boils down, for all general purposes, it is basically a draw. Other than a few FPS in either way based on the hardware setup, it all comes down to your own personel preference. I have seen many benchmarks where they are equal, and I have seen many benchmarks where one card will out perform the other bay large margins. If you are looking for actual results, you can find them either at sites like www.hardocp.com
both are reputable sources.