My system runs this fine..

[=GI=Joel]

I seem to have very reasonable performance. at all high settings. 2xaa. not that AA looks like it has been enabled correctly.

I7 920oc to 3.8
8 gig ram
2wd 250gig in raid 0
ati 69502gb
11.4 beta drivers from guru 3d
seems to me if you start with medium settings it plays like a slide show.
I went all the way down to low everything.
When I put all settings to high except for enabled roads and grass it played very well 30 - 60 FPS.
might consider trying to lock fps at 30 ? might help the stutters ?