Well two 660s in SLI will give you better performance, but they will draw more power, and some games are not optimized to run on two graphic cards, and will not use their full potential. Battlefield 3 is optimized so you will have some great performance there. However with multiple GPU set-ups you may run into driver issues, and microstuttering. A single gtx670 will run all of your games maxed out, and does support 3d. It can also be overclocked to attain performance levels of a gtx680, for $100 less. The dual 660's will offer more performance, but a gtx670 is really all you need. Also in the future you could just add a second one for more performance, but that won't be necessary for quite a while
What are your system specifications, and what monitor/resolution do you plan to game on?
Hope this helps!