Chances are the GTX 660 in SLI will still beat a single GTX 760 Ti.
This is just my opinion, but honestly I wouldn't really do SLI. I've done it, and while I did like the performance I was getting, it just didn't seem all that glamorous with the extra power consumption, noise, and heat. If you're running a 1080p or so monitor, a GTX 660 should buy you into at least when NVIDIA's Maxwell comes out.
Also SLI doesn't combine memory.
Thanks for the reply and input. Would a 750w OCZ PSU be enough to power a 2-way GTX 660 SLI configuration? My other components will be a Haswell i5 CPU, 16GB DDR3 2400Mhz Memory, Blu-Ray Burner, 2TB Toshiba 7200RPM Hard Drive, and a 500GB Samsung SSD. I'm leaning toward the SLI configuration at this point because of the price/performance ratio being in favor of it. I did some browsing on the web and it seems that a GTX 760 Ti will be as powerful as a GTX 670 and that dual GTX 660s should be slightly faster than a GTX 680, so basically a GTX 660 SLI 2-way setup will be substantially faster than a GTX 760 Ti. I don't know how accurate this information is though. I'm willing to put up with the noise of SLI as the particular cards I'm getting are said to be rather quiet, and heat isn't that much of an issue as the cards will be housed in a Corsair Carbide 500R with a 200mm side fan and 4 other fans as well as a water cooled CPU. Power consumption might be a problem, however, which is why I asked if my 750w PSU will be able to handle both cards with my other components.
It's a shame SLI doesn't make use of both cards' memory. So basically a game running on both cards will still only use 2GB? Once again, thanks for your reply, Latios.