To be fair, two of either of those cards will run BF4 pretty well. Also neither of them are energy hogs so a nice reliable 600w psu would do the trick with two of them. The difference between the gtx760 and the gtx660 ti is very small, in most cases they perform almost identically. If you already have a gtx660 ti then I would say just run with two of them for now and call it a day. If you absolutely have to bump up to a new card, I would highly recommend making the jump the the gtx770, as the price gap is pretty small between the two right now and it is quite a bit more powerful.
I'm gonna cast my vote for dual GTX 760's in SLI. That way you'll minimize any potential problems with SLI (blah blah blah lol), and you'll get great performance either way. Also, since you're considering a tri-SLI setup, I'm assuming you'd have room for a third GTX 760 down the road. Doing so would really put you ahead of the 660's. I'd say you'll need at least a 750w PSU, if not 800 or better; overclocking or not.