the only thing as really springs to mind is better/More RAM.
8 is fine, but 16 is better, and it is fairly cheap.
I would not recommend buying a GPU atm, the 970 is great still, i would wait until the card after the 2080, and then buy that, but until then i would not bother upgrading (i use a 970 myself)
Personally i like to have more SSD's, i currently use 1 120 SSD for Windows, teamspeak etc. as my C-drive, and then i have 2 250GB SSD's for games, and 2 1TB HDD's for everything else.
While that is definately a bit overkill, then the principle of having a big SSD just for games, where no other things are on, are very handy and makes it easy to install/uninstall while keeping junk away, so with that in mind, i would recommend buying a SSD drive of 250 or more for games, while keeping your current SSD and HDD.
your CPU is pretty good (i use the same myself) but ARMA are very CPU intensive, and if you have 900 USD to throw away for upgrades, i would recommend atleast looking at the new things that Intel is putting out. i9-9900K etc for complete overkill.
the only other thing as really springs to mind is that your monitors are only 60Hz, so consider getting something with atleast 120Hz.
Power supply is good, keep that
Case is good, only change if you want something new to look at, or the condition of it is bad
Motherboards i am not a expert on, so i wont comment.
CPU Cooler looks acceptable, however, if you buy a new CPU, atleast consider that your current might not be enough, through it should be.