I actively play Arma, should you need someone to game with don't be afraid to hit me up.
I built my own pc, came out great. I can play most games on max; Skyrim, BF3, Fallout. I spent around 1000$ on it. I should point out, you won't find a computer for under 700-800$ that can run the games you like on High/Ultra. Arma is extremely CPU intensive and would require a powerful CPU & Motherboard. You'd also need a great graphics card to get the quality you'd like, and from there you'd be matching quality with all parts of the build. What I mean is: Don't spend all your budget in one place. You need a well balanced machine or you won't get a good performance. I had a friend who duked out for a great cpu and some amazing auxiliary features. Completely forgot the Graphics card and couldn't afford a good one.
I'd be willing to give you pointers or advice on builds if you'd like to discuss via pm or skype.