I thought I had some issues with Arma2 last night, I run 1900x1200 with the same ATi card 5870.
Tho I found I had the view distance set at 10,000
was getting about 25 fps.
Dropped back to 3,000 and was getting a nice constant 35-40.
OA on the other hand is showing the FPS increases I'd hoped, I'm expecting it will mature along the lines of Arma2 in due time