I don't know how to switch motherboards and I can't afford a decent CPU from Intel so I thought that a AMD CPU for 270$ was worth a shot.
While playing ArmA 2 earlier today I noticed that my first two cores were maxed at 2,6 GHz while the other four were idle. This would mean that an increase with 2*600 MHz hopefully would let me reach higher framerates.
I've ordered a AMD Phenom II X6 1090T 3,2GHz Black Edition, let's hope it makes a difference.
Any further advice is still appreciated.