I would just go with the xfx 6800gs, my friend has it, its way cheaper than the 6800gt unless something changed. It is overclocked and runs faster than the 6800gt .. i cant remember if its only pci-e tho :/
Anyway agp and pci-e have about a 1-2 fps difference at this point, so pci-e u cant really justify upgrading unless u already need a new motherboard.
Read some reviews/benchmarks.. they tell the stories
-Msi Neo Platinum 2 Nforce 3 Ultra
-Amd 3200+ @ 2.6ghz (260fsb x10 4xhtt)
-Ocz EL Platinum Revision 2 (3 3 3 10 @ 260 =DDR520)
-Ultra X-connect 500W Modular Power Supply
-BFG 6800gt OC @ 420/1.1
-Western Digital Caviar 200gb 8mb Sata Hard drive
-NEC Black DVD Burner
-Logitech X-530 5:1 Speaker System
-ViewSonic G90FB Black 19\"
-Kingwin Mutant X Silver/Gray w/ window
-Thermalright Xp-90 Cpu running Idle @28c Load @ Highest 51c Average 39c
-Windows Xp Home Edition Service Pack 2
**Whimmy wham wham wozzle!**