In all honesty, you're not going to get much better on AGP. Your 6800GT sports 16 pipelines, so does the 7800GS. The clocks would be higher, but in all honesty, if you don't have less than a 6600GT on AGP, it's not worth it to upgrade to the 7800GS.
If you really decide to, they're going for almost $300 USD even on newegg. If they ever drop to below $250, they will be much more reasonable upgrades for users on powerful AGP systems.
I got a AMD 64 bit processor 3.5 GH, 2 Gigs of Ram, and Nvida 6800 GT. I am playing Lord of the Rings Battle for middle earth II. I am trying to decide what would be my best settings to play on, cause the game is a great game to me, but the units seem to move a little slower to me. Lol, or is it just me. Right now I have my regulater settings just set to Ultra High and 1600x1200.
You can upgrade to a pci-e mobo and video card. Right now you can purchase a good mobo for ~$100 mobo and a 7800gt or x1800xl for ~$300. ~$350 for the x1800xt 256mb. Waiting, its rumored that the 7900gt and the x1800xt 256mb will be priced at $299 shortly (mid march?).
Your systems sounds compatible to grade to a pci-e mobo.
I have a P4 3.4EE, 2gigs of RAM, SB X-FI and a BFG 6800 GT OC. I bought a BFG 7800 GS OC this week. I put it in last night and was able to squeeze a little more performance out of it by overclocking it from 400/1250 to 425/1300. I could probably overclock it a little further but it is very stable at 425/1300.
I don't play any RTS games except Age of Empires 3 sometimes. I only play FPS (COD, COD2). I was not suprised by the modest increase in performance of the 7800 GS versus the 6800 GT. The main reason I "sidegraded" was the cooler running GPU, CineFX 4.0 and the SM 3.0 support. But, as an example of the difference in performance, I am able to play COD2 with DX9 rendering, AF on, 4xAA, extra texture levels, shadows on and world only smoke softening at 1280x800@72Hz. My fraps are between 45 and 65. With the 6800 GT OC, those same settings would not produces fraps above 35.
If I change rendering to DX7 and crank all the settings up, I can play COD2 at 1440x900@75Hz and fraps are pegged at 125. Also, IMO, the image quality with these settings is noticeably better than the 6800 GT.
So, for me it was a worthwhile upgrade. I could have stayed with the 6800 GT but I stretched my rig out another 6 months for a little over $300 dollars. I'll sell the 6800 GT and an X850XT I have and recoup some of that money.
The new box I build at the end of the summer will definitely be Intel 955x or nForce4 with SLI/PCI-e but this current motherboard/CPU setup has allowed me to make inexpensive upgrades like this for two years. I'll be glad to get away from the Socket 478, AGP setup.
Upgrading an AGP system with new parts is a waste of money. In other words, like what the other user said: get away from AGP with that money, and save a little more if you need to.