Ozzy is right, Arma is all about CPU and it is a single-threaded application. Intel chips have far better single-threaded performance, like insanely better. You'll want an i5 or an i7, the higher-tier of both of these CPU families will run similarly. I recently upgraded from an AMD 8350 8-core 4.0 GHz to an Intel 4790k 4-core 4GHz and I experienced a good 50% increase in fps. I get 70-90 fps outside cities and 45-60 inside big cities, depending on how many players are connected. I paid $300 for my chip and it was well worth it for Altis Life.
Trust me, you'll never go back to AMD.
*oh and also you don't want to bottleneck with any other parts, so get a decent GPU (maybe even a 980 with your budget, but don't forget Arma will utilize the CPU a lot more) and a newer Intel mobo, if you care that much about quick rendering in of objects and map-loading, get an SSD.