660 SLI would be a slight better than a 780 (like 5% better?), however more heat, more power and some games that dont support SLI profiles will only run with 1 card. Its the cheapest option if you intend go there. If you have some money to spend you could try and sell your 660 and get a 780.
Switching a 660 for a 770 is like more 40% performance; going from single 660 to 780 is on general a 85% performance I believe. Rememebr SLI 660 you will get around 90% boost (only on good SLI profiled games), but with the issues above, the 800W PSU should suffice that SLI.
I have had 560TI SLI, and had not much problems with games, only on older titles and some with starcraft2. However the heat on top card was intense (reach around 93ºC) and the bottom one going to 75ºC and I have a good airflow in my case with 4 fans, The power consumed was bigger of course, but at least I had a performance similar to a 760.