Arma 3 depends heavily on single thread performance, the GPU doesn't matter that much, but even a 750Ti can run the game on Ultra paired with a good CPU. I'd suggest getting a 1050Ti or a 1060 3GB/6GB if you want to play some more GPU oriented games on 1080p (remember 1050Ti 4GB < 1060 3GB < 1060 6GB). Now for the CPU i'd suggest an i5-6600k (non-K if you don't want to OC) or an i5-6500, but don't go lower than that. You can check out this single core performance diagram:
https://www.cpubenchmark.net/singleThread.html . You will see i7s on the top, personally i would not get one just for Arma, it might give you some extra fps during heavy loads but it's not really worth the extra 100$.
EDIT: Forgot about the RAM
, get 8GB of RAM, Arma won't use more than 4GB so getting more than 8GB won't benefit you in any way. Some people claim to get better performance using higher frequency RAM, so i'd suggest spending a few more money to get some higher frequency DDR4 sticks.