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
Intel Q9450 ATi 5870 Corsair 4gig Samsung 245B 24" G15/G5 - Antec 900 Gaming Rig