One good GPU like a GTX 780, GTX 980, R9 290/290x is always better than two mediocre GPUs. I actually used to own 2x GTX 660s in SLI. The price to performance ratio back in 2012 was actually worth it as you were getting just over GTX 680 performance for over $100 less with 2x GTX 660s. Now, though, you can get the same performance from a GTX 770 for about $80-90 more than one GTX 660 costs. Take a look at this GTX 770:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121770&cm_re=GTX_770-_-14-121-770-_-Product you would be able to use a cheaper motherboard that doesn't require SLI, you will have a better VRAM bus and it would be more compatible as a single card usually has less driver issues than SLI or CFX. If it were me, I'd spend that money on a GTX 770 or even a GTX 780 which are coming down big time in price. A GTX 780 can be had for just $310:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121779&cm_re=GTX_780-_-14-121-779-_-Product which is almost the same price as a GTX 770 and gives you even more power than a GTX 660 SLI setup plus even better VRAM, etc.
In summary, 2 medium GPUs in SLI < 1 Good GPU
Performance: GTX 780 > GTX 660 SLI > GTX 770
Price:
GTX 660 SLI = $380
GTX 780 = $310-ish (rebate)
GTX 770 = $290-ish (rebate)
Best value right now - GTX 780 for that price range
EDIT: I should also mention that since the GTX 660 is a mid-range card, you can only SLI up to two cards with it. The 770 and 780 can do up to 3 cards with the right motherboard and power supply. This obviously limits any future upgrades not involving replacing both GTX 660s and buying all new GPU hardware while a single GTX 780 can easily be upgraded in the future by adding a 2nd or 3rd if your motherboard and PSU allow.
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