That system should do you pretty well. One thing that i've found helps was something that someone here at work sent out...

Try setting Sound Detail to Low and enabling sound Hardware Acceleration.

I have a feeling that this will help many people. For me it had a more noticeable impact on performance than changing any of the graphics options did. Before doing this, my framerate would have large and sudden dropoffs when I got into the middle of intense action. They'd be so bad that the game would get choppy. I run a 1.4 ghz Athlon with 512 mb ram, GeForce 4 Ti4200, and Soundblaster Audigy soundcard on Windows XP, all latest official drivers.

This also worked even more noticeably on my friend's system, which is slower and has a less powerfull soundcard (Soundblaster Live).

I suspect that either there's some kind of bug or the sounds don't offload onto soundcard hardware very well, especially under higher detail sound settings, thus shooting cpu usage way up at times.

Audigy's normally have around 3 or 4 % cpu usage I think, but on this game you can definitely see a difference in framerate when you have high detail sounds on, get into a large battle, and choose the Disable All Sounds option to compare the difference.