I'm very un-experienced when it comes to computer specs, graphics cards, etc..
One of my buddies just recently bough an Alienware x51 R2 and said it runs every game he's played so far (Arma 2, Battlefield 3/4, Crysis 3) on Ultra/High graphics at around 50-60 FPS constantly.
I can run Battlefield 3 on High at bout 50-60 FPS, but on BF4 my FPS drops to about 40 or so on high. Arma is just a whole nother story, I get like 20-40 fps (if i'm lucky) with my graphics around medium. I've seen a lot of people hating on Alienware, and i'm guessing because they're so pricy. My budget is around 300-800. I know I won't be able to get much, and if I have to i'll just upgrade what I have now, here are some basic specs of my current PC.
Graphics Card: AMD Radeon HD 7870
Motherboard: (Wouldn't show up in DXDIAG)
Processor: Intel(R) Core(TM) i5-3350P CPU @ 3.10GHz
RAM: 16 GB
I hope that's enough info to go on, if not, I can provide more, just tell me what you need.