im pretty sure that arma2 is largely cpu dependant, and the cpu side of that APU is pretty meager. the game might run, and depending on resolution/setting it may even be playable...but a GTX780 will be massive overkill and i would expect any APU to bottleneck pretty severely on that game title with that video card.
plus, youd likely need a new PSU. that card can draw a pretty good bit of power
that being said, if you had enough PSU for a780 it may be worth buying now and upgrading mobo/cpu later down the road