Usually I change my video card every 2 or 3 years, by doing so I usually double my performance with each upgrade. But from a 660-Ti to a 670, the little performance gain might not justify the price. May I ask why you want to upgrade? Do you have any issues with your 660-Ti?
I would wait for next generation, nvidia series 800 next year maybe? But if you absolutely need to upgrade now, at least I would follow swilczak advice and wait for the 760 to be available in your area. As he said, the 760 performs right on par with the 670 and costs 100$ less.
*** Edit: Yeah, as others pointed out, the 770 would be a more logical upgrade, this would give you a more significant performance boost compared to the 660-Ti, but of course they cost more. But as I said initially, if you can postpone your upgrade to later, you'll have more options and price drops.