I've got a ATI 3850. It can run textures at 200% and AA at 8x, but damn if dynamic lighting doesn't mess it up. So I turned the lighting features to low, everything else at high, and it runs like a charm.
Dynamic Lighting, Post-Processing, Rendering and Anti-Aliasing. At least these are the ones I play with to get rid of choppiness. I don't recommend turning them all off because if you do, it'll look quite crappy.