bast_imret said:6800XT is an 8 pipeline 6800. Most people have had good success unlocking up to a full 16 pipelines and 5 out of 6 shaders.
6800GS OC is a slightly dumbed down GT. It has 12 pipes out of the box, and 5 shaders. Once again, with the AGP version, you can try to unlock the extra 4 pipes and the 6th shader. Started to become a bit more hit and miss in the latter releases of the 6800 series.
6800XT has a memory BW of 22.4GB/s while the 6800GS has 32GB/s. Clock speed is better on the GS as well. For those reasons, even a full unlock of all the pipes and shaders on the XT will still not get you to GS speeds, but it's still a good improvement from stock XT. GS is a better overclocker as well.