XT is akin to the LE (light edition). It is a reduced-performance card.
6800XT performs somewhere in between a regular 6800 and a 6600GT.
At this point, 6800xt is a very budget card that I can only recommend for those on a very small budget or fairly old rigs (<2Ghz P4 I'd say).
6800GT is the clearly superior card, if the price is decent it's still a good AGP upgrade path for an older system.
There are other options in AGP however, including the 7600GT or the x850 series. Often these are priced less than a 6800GT (which should be hard to find in AGP nowadays), but yield better performance.
Browse around the GPU forum to get a good idea of what to aim for, and use Tom's charts to compare relative performance.
Cheers