I too upgraded from a 9800 pro 128MB to a x800xl 256MB. The the difference is ~2x, and even more at higher resolutions. I play BF2 on my first system at 1600x1200, Med settings, no AA, with frame rates typically 50-70 FPS. On my second, 800x600, Med settings, 6x AA, not sure of Frame rates, but very playable for my friend. It would probably run 1024x768, but no AA and most settings on low. Hope that helps if BF2 is your game.
Bottomline: if your unable to play your games, maybe upgrade to the 800xl. I'm very happy with mine. but if you can wait for next gen parts, you'll only get better performance, but at twice the price. and who knows what's to come of AGP. I'm very disappointed in nVidia for their lack of AGP support on the 7800GTX model, but if they want to limit their own market it's their loss. I'm sure ATI has something great in the works. time will tell.