top of the line are the x850XT PE and the 7800GS cards, both are about the same in performance, but the X850 AT has confirmed is the fastest thus far, up to this point.
the other guy is right, there is not really a card above the x800 that is worth upgrading to. I mean, the x800 was top of the line for the Radeon series back in 2004, you're looking at spending more money on something that will get you the same resolution with a few more textures running from low to medium in modern games? a few more frames per second...? that's about it, don't hope for too much faster, the x800 is a good card, 256-bit memory interface with GDDR-3 memory.
save up for a PCI-e card, spend the extra money on the setup. AGP is a dead end solution if you're playing games with it right now, as far as video goes. The power requirements are already too absurd for highend AGP cards, the power draw is far greater than what PCI-e cards can deliver at the same performance.
You can wait for ATI's release of it's next latest AGP card, otherwise the X850XT PE or the 7800GS will do. I still recommend PCI-e for video if you're looking to play more modern games with your graphics card.