I'd say the X800XT would easily be quicker in BF2, and in almost every other game. There is simply no comparison, the 6800GS AGP is clocked lower than the PCI-e version even.
(edit)OK, here you can see
, a X800XL beats out a 6800GT in BF2. A X800XT is basically a faster clocked XL for comparison sake(akthough in this case it's a 512MB), and the 6800GS AGP being slower than the PCI-e, will also be slower than a 6800GT.
So I'd say the X800XT is way better in BF2 than a 6800GS AGP, especially with eye candy turned on.