While I agree that GTX 670 sli is better than one GTX 760 in performance, all of these cards are quite outdated by now. Also SLI is never favourable over a similar perfomance single card like a 960.
I would advise you to look at a 960 or better yet wait for the Pascal 1060 or 1050. Some of these options will be much better than an old SLI setup. The RAM on a 670 SLI will be a limiting factor also. I would get a 960 or if you can wait a bit a 10 series equivalent. Also your motherboard should be OK for a single card like a 960. The 960 requires a min of 400W PSU. The 1060 or 1050 we dont know yet, but it should be even lower.
Gabor