Overall the difference is worth it. The 760 performs pretty close to the GTX 670
One other benefit of the 760, is the ability to overclock much higher. Many 760's allow a 12mv voltage boost in addition to having more overclocking headroom. I currently have my card at a max clock speed to 1306.5MHz for the GPU (extra 326MHz on the GPU over reference), and an additional 350MHz memory overclock over reference. Screenshot of my GPUz http://i.imgur.com/z4QCoi5.jpg
Here is a random 3D mark that I ran with my card http://www.3dmark.com/3dm/2321352
The GTX 660 is not worth it. If you want to save a few bones, and still be able to play just about any game at medium-high settings @ 1080p-- go for the GTX 750 Ti, otherwise the GTX 760 at $219 is well worth it with the recent price cuts. ( i paid 249 for mine before MIR)