Another Asus board probably. They're always a good choice. Obviously you need SLI. A good P45 board would serve you well if you want to OC the Q6600, but it would not have SLI support.
Stay away from 680i nForce boards if your goal is to OC your Q6600.
To do SLI on a 775 without 680i, you need the newer 7 series nForce.