go for a single card because of the drivers.
lets say you have a single 660 and get 100fps in a generic game
in a perfect world scenario with a second 660 you should be getting 200fps but this never happens. games must be optimized for multiple gpu solutions...some are better optimized while some are not...with new games you will have to patiently wait for new drivers to come out to improve the scaling between two cards while with a single card you wont have that issue. also two cards take more power than one, one card takes less space for future expansion/upgrades, one card generates less heat than two, two cards will generate more noise than one