GTX: 970, best cost effective graphics card In my opinion, although works much better for other games that aren't arma, as arma is mainly cpu(as you probably know). There are different brands that also tweak the GPU's and add their own touches like performance, cooling, size, etc so find the brand that has the specialties you desire. These brands include MSI, Gigbyte, Sapphire, etc. RESEARCH IS KEY TO BUILDING A MACHINE YOU WILL LOVE!!!!!
CPU: idk something i7, no need for the newest shit I mean I use a Intel i7 4770k that is like 3 years old with my 970 and I get 35-40 in cities and 60 everywhere else
HDD/SSD: big one, buy a good size ssd, maybe 250 or 500gb to load your OS, and key programs and games, I have my HDDs on raid 0 just to get the max out of them with my other games I don't want to clog my ssd with. SSD WILL INCREASE EVERYTHING SO IT IS A MAIN PART ALONG WITH THE CPU; increases anything from boot times, load times, less texture bugs, more frames, etc. Anyways brand of HDD doesn't really matter a TB can go for like $50 and the SSD is the real one to pick out. SSD prices range significantly in storage space so anything larger than 500gb with be $250+ which is a lot to spend on a SSD but if you have the extra money and want to put everything on it go ahead. Also I recommend researching raid as you can pickup a couple TB hard drives and set them on a desired raid and it will pretty much act as a collection of HDD's put together to make one big ass SSD, depending on what type of raid you have.
Power Supply: Use pcpartpicker to make sure you have sufficient power supply
Monitor: 1080p and 60Hz monitor atleast, would not recommend in the slightest any lower or it completely undermines everything you've bought above
Motherboard: personal preference, research motherboards and motherboard companies and find the right balance for you, upgradable vs cost vs features really. Personally I use MSI because I really like their features, bios, overclocking, and support.
Keyboard and mouse: personal preference as you really should pick out what you'll be resting your limbs on for hours on end lol