Unless you want to buy a new motherboard as well, to pair with an i5, you might consider adding a good CPU cooler and just overclocking it up to a more respectable level which will help somewhat. Kind of depends on what motherboard you have as to whether overclocking is a good idea or not.
Certainly be much cheaper, although it won't gain you as much performance, than going to a new cpu and board.
Not to sound stupid, but you do have your display connected to the GPU and not to the motherboard right?
Honestly though, I've talked with several, at least five, other members this week on Tom's that have similar issues, with frames good and normal, very high sustained values, on other titles, but in Arma they drop down to like 20fps in multiplayer. 60-80 in multi on other titles. Upgrading, even to an 4790k i7, might not fix the issue.