Hey, i had the same problem last week, couldnt decide on my AGP card...
I was looking at the x850 XT and 6800 GS and decided to go for 6800 GS OC.
with my old card (radeon 9550) i got a 3dmark 03 score of 2100.
after the upgrade to a 6800 GS OC, the scores went up to 9860.
I also play bf2, used to be low GFX, 800x600, but now i can easily play on medium settings and on 1280x1024 with no lag. High settings are possible but it is pointless as all the shadows and stuff makes it hard to spot enemies.
Also, quake 4 with max resoluiton (my monitor goes up to 1280x1024) and Highest settings, and it is also perfect. So is NFS Most wanted on maxed out settings.
I think 6800 GS OC is a good upgrade for your AGP. I think it will last me at least a year or more untill i get a PCI-E system.
P.S i think 7800 GS is well overpriced compared to the performance. it would also be pointless to spend so much $$ and then upgrade to PCI-E as you will eventually have to.