gtx 660 ti SLI vs GTX 770

Solution
More specifically,

1) A GTX770 is only about 26% faster than a GTX660Ti: http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_770_Direct_Cu_II_OC/27.html

2) Of the many games chosen for a review, 2xGTX660Ti was 70% faster on average (at 1920x1080) than a single 660Ti:
http://www.xbitlabs.com/articles/graphics/display/geforce-gtx-660-ti-sli_6.html#sect0

*Thus, going to a GTX770 makes little sense and you would ONLY benefit slightly (26%) if the game didn't support SLI.

These numbers assume similarly performing cards vs stock.
More specifically,

1) A GTX770 is only about 26% faster than a GTX660Ti: http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_770_Direct_Cu_II_OC/27.html

2) Of the many games chosen for a review, 2xGTX660Ti was 70% faster on average (at 1920x1080) than a single 660Ti:
http://www.xbitlabs.com/articles/graphics/display/geforce-gtx-660-ti-sli_6.html#sect0

*Thus, going to a GTX770 makes little sense and you would ONLY benefit slightly (26%) if the game didn't support SLI.

These numbers assume similarly performing cards vs stock.
 
Solution
Indeed the 660 SLI will be quicker athough to be honest you do sometimes run into random issues with games which can be annoying. If I were you with the funds you may have I would just buy a single card (Best one you can afford) as you will still have great performance but won't have these random issues.