Need help deciding on a new GPU (Gtx 660ti, 670, or 680?)

My budget is going to be around $500-$600 dollars.

I was really thinking the 680 4gb instead of 2gb but I'm not quite sure if I am going to need the extra ram. I use two monitors but only one for gaming at 1440x900. I play DayZ, Battlefield 3, and other latest games and whatever is coming out. (Battlefield 4 etc).

Because I need to get a new quality PSU I don't know if I will be able to afford a 680 4gb or a 2gb. I was looking at the Radeon 7970 and was wondering what would be the difference besides I will be saving money. Same with the 660ti which I heard you could overclock it to have the same performance as the 680. (Is that true?) And what would be the big difference of getting a 670 4gb or 2gb opposed to the 680?

I could really use some help on making my decision. Right now here are my specifications-

Windows 7 64 bit
500gb HD + 200 Gb external
700w psu ModxStreamPro
Phenom ii x2 960t Quad Core overclocked @ 3.2 ghz
Sapphire Hd Radeon 6850 1gb edition (have two but crossfire has too many glitches and performance issues)
Asus m5 a99x mobo