So i was having the same problems with poor FPS even with low settings when i stumbled onto something from another forum (cant remember what forum:bounce3:).
anyway set your interface and 3d res to your desktops leave the game then enter ("C:\Program Files\Bohemia Interactive\ArmA 2\arma2.exe" -window) withought the brackets into the desktop shortcut target.
windowed for me i can play with great FPS everything very high, ive just fiddled with the box to get it to fit the screen.
Seems theres something very wrong with fullscreen.
