Your motherboard will not support sli so make your video card choice a good one. As a side note, my dual msi gtx 460 1GB, i7 965, 12GB DDR3 2000MHz RAM at 1867MHz runs BF3 on ultra at 55-60 FPS with AA on and two monitors running at 1920x1080. I have to OC both cards to 850MHz to get this FPS. Stock speed of 725MHz only gives me 45-50 FPs on ultra. I have not tried Nvidia 3d vision so I cannot comment on that. Any hardware comparisons I have read and there are a lot of them on Tom's Hardware as well as many other sites put dual gtx 460 1GB slightly ahead of a single gtx 580. Here is one example. http://forums.anandtech.com/showthread.php?t=2118943
Two 570's would be plenty but you need a new motherboard to run those. Not sure what your budget is but a single gtx 570 is going for anywhere from 450 to 600 on ebay. With a new motherboard, this is gonna cost you a few bucks!
If you want to do this cheaply spend 150ish on an sli ready motherboard and another 150ish on another gtx 460 and you will outperform a single gtx 570 and you get a new motherboard out of the deal.
Almost forgot, for simple gaming like BF3 my i7 965 typically only runs on 2 of the cores at no more than 75% use which still leaves 2 cores and 4 threads. Since mine is essentially the same as yours with an unlocked multiplier and a faster qpi speed I doubt you will run into any bottlenecking from your cpu. My RAM usage does peek at 5.4GB while playing BF3, if you are wondering.