Do you need an operating system? Trying to fit an operating system into any budget build is not fun. I'd look at transferring the one you already have across if that's at all possible. For example, without having to fork out £50-£70 for an OS you could get something like this -
CPU - Arma is quite CPU intensive so you'll need an i5 minimum really. It's quite badly optimised and tends to run mostly off one core.
i5-4430 3.0ghz - £120.
Motherboard.. if it's just for playing games, not over clocking or doing anything out of the ordinary then you've got no need to spend more than £40 on one. Go for one of the H81 mobos and look closely at what features it actually has - don't just look at the cheapest one and think it's...