It depends what you want to do in Arma 3. Big multiplayer servers, especially modded like roleplay or wasteland you will probably lag.
I have an i7 2600 and a 1060, when i upgraded my 560ti to a 1060 6gb i noticed 0 change in fps what so ever in the same spot on a single player armory map that i tried. So your 960 will be fine but i'm not sure on your cpu.
I know someone with an older i3 who says he gets about 20 - 40 frames on a popular/populated server and 40 - 60 on around 20 players. definitely more around 20 in a big town.
Be hopeful! The arma series is not very optimized though.