An i5-2500K with a decent s1155 P67 mobo and (at least 4GB) ram will be a great upgrade. It'll allow for greater viewdistance and keeping up performance with loads of ai in the area.
A bigger gpu will get you more performance with "eyecandy-settings" all kinds of motion blurrs and stuff. But an 5870 is still a pretty good card for arma as long as you dont crossfire them. An expensive gpu upgrade (gtx 580) will get you a gpu that's about 40% faster? dunno, it's a guess. Anyway, upgrade the cpu first.