6800 GT of those two - why not 7300 GS??

I enjoyed my PCI-E 6800GT, but for AGP there seems to be a power problem. As AGP cards do not have the additional AGP->PCI-E bridge chip near the GPU, those cards run ~10°C cooler then PCI-E models. But still, PCI-E overclocks better then AGP models, due to the power problem.
The ATI cards are very fast also, but I don't know if there are problems there.

The X850Pro is a tiny bit faster i think.

http://www.hwbot.org/quickSearch.do?hardwareId=GPU_140
http://www.hwbot.org/quickSearch.do?hardwareId=GPU_269

I would go for 6800GT, it's tried and proved to be quite the contender and has a technical advantage over the X800, 6 series supports SM 3.0 which is needed for some games (like Oblivion) to be able to play with HDR "on". The X800 series is lacking here and some games look less interesting.