Upgrade! Pls help me :)

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1

Operat: Win 7 32bit (6.1)

Mobo: Z68p-ds3
Proc: i7 2600 cpu (3.40ghz)
Ram: 4096mb 
Gpu: nvidia geforce gt 520 (2346mb)
 
My computer is pretty crappy, i have to admit. It lags on most games and i try to play arma 3 but i have to change all the settings and put it on a super low resolution to make it playable. I was just wandering if you guys could suggest what parts to upgrade and what to. I have a budget of around 700$ and i want this machine to play at its best. (I know that my operating system is bad and ill change it when i upgrade it {not included in budget}) so please tell me what you think i should buy and thank you in advance! :)
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First of all what is the resoulution you are playing on?

AS well what PSU do you have?

The only thing you could change with your PC is adding more ram (8 GB overall ought to be enough), and buying new gpu. 780 gtx, 770 gtx or 7970 are very fine choices. and with 7970 you get a bunch of AAA games.

But i cant tell you anything unless i dont know what PSU do you have.