660ti is joke, don't even consider it. It is under powered from the begging, offers lackluster performance and is over priced. Next to the GT640 the 660ti is the second worse card Nvidia has this gen from a price/performance ratio. Stock the 660ti is about even with a 7870, with the 7870 tending to have higher min across the board. When Overclocking is involved the 7870 kills it. I would recommend getting the gtx670 if nvidia is more your game, much better deal. Though you could get a 7950 and overclock it, at 1200mhz it equals a 1300mhz GTX680 so thats something to consider. I honestly just can't recommend the 660ti though.
In SLI the bandwidth kills the performance at higher rez making SLI kinda pointless as why go SLI if you cant max out AA. SLI 660ti is a bad idea IMO