I had a Phenom II x4 965BE overclocked to 3.9GHz and I still got fps drops in Arma II and DayZ. I'm telling you it's the CPU and it is a bottleneck. (I should note I had a 1GHz GTX 560 TI at the time and upgrading to a 670 did almost nothing to improve the fps). Not trying to be mean but it is a bottlenecking issue courtesy of the Phenom, switching to a 3570k alleviated the bottleneck for me. I'd recommend either saving up to upgrade or just disable anti-aliasing to try and boost the fps. I mean this is the nicest way possible, Phenoms are becoming outdated for demanding games like Arma, BF3, and Crysis, sorry.
I found the TH gaming CPU heirarchy chart for you just to see where the Phenom compares...