First off, the 260 is not running physics for ARMA. That game is not a PhysX accelerated game.
Secondly, what sort of crash are we talking about? The game crashes? The computer BSODs? Makes a big difference.
Also, when you OC anything there's a chance for errors. Have you thoroughly stress tested the CPU and RAM after OCing? (and how did you do it?)
Have you played any other games? Any issues?
Are your temps getting too high?