I would say that he spend the xtra bucks and buy the 7800GS as that will have considerably better FPS, I believe, than the 6800GS. ...The GeForce 7800GS definitely has serious horsepower and easily beats anything previous AGP offering from NVIDIA.... From an article at THG Graphics.
~Ibrahim~
P.S. Currently, the fastest ATI card is the X850XT PE. I still say the 7800GS, as it has that shader thingy version 3.0, while the X850 doesn't. Kind of is the problem that was with BF2 in the beginning. We few people with GeForce Ti 4600's were the step between 1.3 to 1.4. The next card had 1.4, think it was the FX 5200. (I'm excluding the 4800)
Article:
http://www.tomshardware.com/2006/02/02/nvidia_7800gs_confirms_agp_aint_dead_yet/