One thing that helped me when I had studdering issues, was to turn down lighting to medium.
The reason for all the studdering is, it is constantly using the page file.
And when I had 1gig of ram it was using 1.2gigs of my page file, and using 800mb of my physical ram.
Now since I have 2 gigs, the page file is hardly used, and I am able to to run everything at High Settings, 4xAA, 8xAF, 1600x1200 @ 85hz.
Game looks and plays flawless.
Now if we can only do something about people spawn camping for a plane or heli.