I just upgraded my rig with the same stuff, less RAM than what you've got. I'm pullin about 2500 on 3DMark05, I think I topped out at around 2700 ocing my 9800. I had the same problem you did at first. The MAIN priority was upgrading ALL the drivers. The ones that came with the board were way outdated (as always), and mine is a rev 2 board.
Gather up all your drivers, update, and see what shakes.
Another thing I had to do was make sure my VGA was on it's own IRQ. That helped.
I had to chenge the AGP settings (and almost everything else) in the bios. I also upgraded the board's bios.
The whole affair has been a real joy, and I'm still not happy with the numbers, but they're average, and what I should be getting. The only bios performance tweaks were to raise the FSB by 15 and go to turbo.
One of the first things I had to change was the ram setting to ddr 400. Default was a lot lower.
I got a lot of help at another forum that made all the difference in the world. If you don't iron it out, I'll post a link to the thread.
Hope this helps!!!
Scott