for the beginning I could recommend a thread Jorrit started some time ago:
for 64/128MB vid cards I guess you use recent drivers for all components (chipset, vid-card ...) and you have proper system settings (like AGP 8x turned on in the bios etc.)
I know there is some lag once in a while, but on my 1533MHz Athlon-XP, 512MB Ram, gf3 the game runs smoothly about 40-50 fps most of the time.
I use linux not win, but with your specs there shouldn\'t be much lag anyways.
Did you ever run any benchmarks ? Do other games run smoothly ?
Edit: One more thing try to deactivate all those little things running in the background (like cdrw progs, p2p (point to point, not pay2play here..

) clients and so on, only run the most necessary things (like a virus protection)