from 660 ti it wont be a much noticeable upgrade to a 670
You can go for SLI or if single Card a factory overclocked 3GB 7970 or even a GTX Titan if you can fund it.
Single card is better but SLI is stronger
660 Ti SLI > GTX 680
but sli brings issues
more power consumption
more driver issue
some game doesn't have sli support so 2nd gpu doesn't kick in.
Its a matter of personal preference.