You know.. from what ive been reading, it seems like the game is CPU limited. I say this because you can usually turn on AA 8x, and have almost 0 performance drop. If the videocard was the limiting factor, there would be a much larger fps drop. Also along the same idea, the resolution doesn't matter. 1024x768 vs. 1920x1080, the fps are (more or less, w/in 5fps) the same. Both these factors seem to point toward a CPU bound game.
Having said this, I usually get 50-60fps in normal, and it drops to ~30-35fps in either of the other two modes. I have an AMD x2 4400+ @2.2ghz, although I overclocked it up to 3.0ghz, and there wasn't much of a change. I think if Dylan thinks about what is going on processor wise in the game, he might be able to increase the performance a bit.