Unless you're finding a really excellent price on the X800 XT(or the XL, for that matter), they're out of the $250 range. The 6800 Vanilla should perform just slightly worse than the X800 Pro, excluding OpenGL-intensive games(Doom 3) where it will take a small lead.
If you're getting a sub-$250 XT or XL on the ATI side, jump on it.