Arma 3 is a very demanding game. In my opinion from having over 2,000hrs on it I would suggest a computer a GTX 980 or more as the graphics really fuck frames up.
EDIT: Not to mention a good processor.
That's very misleading. The game is CPU intensive and heavily relies on it.
Ram and the CPU is where it matters.
The bare minimum for that game for medium/high settings is a pentium overclocked, AMD FX 6300 overclocked, or any low end i5. The game will be very unhappy if you are under 8GB of Ram. It will gladly accept any GTX 700 series card.
What the game loves is a high end i5 to any i7 (Hyperthreading helps a lot). 16GB of Ram is a healthy meal. Any GTX 900 series card won't have trouble with this game.
You won't want to assume a $2,500 computer is needed to make this game playable. If you optimize it in the launcher settings and game files, you won't have any issues.