I'm sure your mobo is fine for any modern GPU made in last couple of years,i'd rather think about a PSU,you need special 6 or 8 pin connectors and quite beefy PSU like 600-700W i'd say.
But if you want to get a good mobo get Z97 buy an I5 4690k and profit from great Arma 3 performance cause GPU upgrade won't give you that much in this particular game if it's overall upgrade definetly get a new GPU!