personally I think the X800 XT's are too high priced against the 6800 GT's to be worth it. the 6800 GT's outperform the X800 XT's in some cases also, but X800 XT's do win most of the time
6800 Ultra's cost about as much as X800 XT's, and are probabbly a better card all round. the X800 XT performs better in HL2 but not by a lot.
the X850 XT is a better card than the X800's and 6800's, but doesn't have smart shader 3.0 and is really expensive. go for the X850 XT if you have a lot of money to burn, but go for the 6800 GT if you want better value