Don't get any thing from the X800 series (except the X800 GTO2) because:
1) bad drivers
2) only has SM 2.0
3) poor price/performance ratio (again with the exception of the GTO2)
4) geforce 6 cards are better overall
A 6600GT will kick the X600XT all over the place.
Even though the X800GTO2 is a decent card because it can be unlocked to a X800 PE, it still only supports SM 2.0 and it doesn't fix the bad drivers. Go for a 6600GT or a 7600GT.