Keep in mind that you must have a decent psu within ~750W for two 560 ti 448, the 660 ti might be a good choice, since it's hard to find another 560 ti 448, and the price you might pay for it might not be worth it, you can sell the card to cut down a bit the cost of the 660 ti.
Keep the card i will buy it from you be about 2 months. I have a monster corsair HX1050 PSU therefor I can run almost anything. I have a twinfrozer 3 my self. I waiting for the launch of the swiftech H220 I am buying it then it's 560ti 448 sli time.