The 660 ti SC is a pretty solid performer compared to the 670, it just depends on what games you're into:
Looking at BF3:http://www.guru3d.com/articles_pages/evga_geforce_gtx_660_ti_sc_review,17.html
Around a 10fps difference translates to a 15-20% performance impovement using the 670, but if you're looking strictly at fps and want to target staying over 30fps, the 660ti SC can do it.
Now if you look at something like Metro 2033 The Last Refuge:http://www.guru3d.com/articles_pages/evga_geforce_gtx_660_ti_sc_review,15.html
You might end up hitting some pretty choppy spots on the low end of the FPS with the 660 ti SC.
For the most part the 660 ti sc will give you decent performance. If you have budget for it though, I'd grab the 670. Also, you don't need a 4GB version of the 670 if you don't plan to go with a 3-monitor setup which would require at least another matching 670 4GB at a later date. Shoot for something more like the EVGA GTX 670 2GB (stock clocks):http://www.newegg.com/Product/Product.aspx?Item=N82E16814130782
This is a great card and would be a good compromise for you.