Check your ArmA 2 video settings, what is Default memory (default video memory) set at ? If its not set at Default, try that and report back.
Video memory should always be set to Default by the way, it's a bug in the game. If you set it to anything other than Default, it will be limited to 512MB thus not utilizing as much as possible from your card.
Have you always had this problem or did it start to happen recently? If it started recently, have you changed anything with either your settings ingame or in windows ? New drivers, hardware etc ?