I don't entirely agree with data on this isssue;
I reckon the main thing that'll slow down your PC is your RAM (as you have noted) , as opposed to your CPU. Your CPU is a fair piece of kit, that should be able to chomp down on any software. I mean, The latest titles (like Black & White 2) need 1.2ghz. So your CPU still has life.
An upgrade to 768 or even 1gb should speed your PC up considerably. And of course the new GPU will give graphics a much needed boost
With more memory and a better GPU, your CPU shouldn't be a problem
but make sure you've got a decent AGP speed on your motherboard
The 6800 is an 8x card (I think) so ensure you've got an 8x agp slot, otherwise it could severely topple its performance.
But;
When Windows Vista comes out (summer 06), hardware prices are going to increase dramatically, so take advantage of their low cost at the moment.
Good luck with the new GPU
[edit] Also, an article on installing AGP GPU's
http://www.helpwithpcs.com/upgrading/install-video-card-agp.htm