RCFProd :
You can optimise the FX-8320 really well AFAIK for Arma 3. It requires quite a bit of tweaking I guess but you can make it use all of the 8 cores. So yes, imo that's a good upgrade if you manage to tweak it and all that.
However if you have a 760 or 970 chipset motherboard don't get the FX-8320, get the FX-8320E for cheaper and keep it at stock speeds. Otherwise you'll meet a friend called throttling.
Arma 3 wont use more than 2 cores, ever. 1 core for main simulation and 1 for texture loading etc.
The tweaks do nothing. They are applied atomatically these days (vram, ram, corecount).
Intel is king in Arma-games.
Sadly I have AMD. The only way I can find any performance increase at all is to run client+server minimum.
This of course requires me to be the host but I do it this way to play singleplayer missions/my own missions.
Add a headless client and it helps a little too, not by much tho.
Arma 3 still won't utilize CPU fully.
GPU will do nothing. I went from GTX 760 to Radeon 290X, same FPS.
When I play the Campaign, I have roughly 60 fps but as soon as I play online, it drops to 20-25. Regardless of server, scripts and so on, whatever the devs are blaming poor performance on.
You should try the first game in the series, OFP. To this day that game struggles to push out 60 fps consistently. 100 fps in places, dips to 40 for no apparent reason. That's a 15 year old game.