^ this. Blower type cards are best for SLI/ Multiple GPU's. They are louder but they can keep your cards temp down, specially with pre 600 series cards such as 400 and 500 Series.
Worth going SLI on a GTX680 or GTX780? Depends, a single GTX680 should be plenty at 1080p / single monitor for almost everybody. and you could always put in a second card for SLI when the prices come down. Thats what I would do. a Single GTX680/780 now and another one when GTX800 series comes out or next year. Its not like you "need" to upgrade every year or anything. I am still rocking my GTX465's unlocked to 470's on a i7 920.