If you don't plan on putting in another video card within a few months, it's generally not worth it to SLI. The Gigabyte EP45-UD3R is fast, inexpensive, and an all-around great part you could consider for that role.
Also, check out a Corsair 650TX. It'll power one video card very nicely, and Corsair is one of the top PSU makers.
Looks like a good build. What do you plan to OC the Q6600 to?