The X850XT PE is hands down the best AGP card when it comes to performance. The 6800U can't be technically found anymore, but if you find one then I guess it's good too, although I think it's overpriced compared to the X850XT PE.
The 6800GS is a great alternative too, and supports SM3 unlike the ATI generation 2 cards.
EDIT: Seems many sites have removed the X850XT PE AGP from their sites too, so I guess the 6800GS is the best contender.