do some reading before you just 'post'. You obviously didn't try to look yourself.
6800XT is a crippled 6800 card sporting 8 pipelines and varying amounts of memory.
6800GT is the older model of a higher end 6*** series card with 16 pipes and 256mb of GDDR3, I think.
6800GS is the revised semi-new 6800 series card that has 12 pipes, but is a revised version. It performs on par(-ish) with the 6800GT.
Ryan