get the 680,You will get a few less frames per second BUT get a good 680 with an aftermarket cooler and you can overclock that like crazy...The 660's in sli are good...if you dont mind working with SLI problems,More power and more heat.Most new games wont work with sli properly till they have an update.But a single 680 will work OR if you want get a 7970 Ghz...its the single fastest single GPU out there to date.
I assume you ment 680. You can find it just about everywhere sli 660 beat a single 680 by about 10 to 15 fps but go with 680. 1. Because sli 660 will make a lot more heat. 2. You can always add another 680 down the road for some serious gaming power. Always buy best single gpu you can afford so you can sli that later. I would suggest msi (I ahve msi 680) cools extremely well oc well and oc easy. Also asus 680 cools very good also.
Overall unless you really need the additional FPS for something like multi monitor gaming a single card is a better choice. With a single card you have less heat, lower power consumption, less driver issues (some games don't support SLI on release), and less problems in games.
Also I would not go with a GTX680, instead I would recommend a GTX670 as it is $100 cheaper and has a very similar performance to that of a gtx680. Also if you overclock a gtx670 it can match or even exceed a stock gtx680 for $100 less. I would get a non-reference gtx670, one with an aftermarket cooler, like a Asus direct cu, Gigabyte windforce, or MSI power edition card.