I play a LOT of arma3 , see my sig for my Spec
I game at 1920 x 1200
New GFX card would be a good choice
In what configuration is your RAM (how many sticks of what size)

@Op did you buy your ram all together at once or 2 seperate kits ?
I had issues with 2 x 4G , then buying another 2 x 4G (same make same model same manufacturer , that would not play ball together ended up selling both and buy 2 x 8G kit , worked great
As regards OCing your CPU , switch off HT before trying it and see if that helps