First off, no point in getting 3 sticks of RAM. Sandy Bridge is dual channel, so you'll want to go with 2 or 4. 2 (8GB total) will be fine.
The 850W Corsair TX will be fine for that system, even with SLI. See here: http://www.extreme.outervision.com/PSUEngine . I got 781W min, 831W recommended with conservative component estimates and 20% capacitor aging, which you likely won't ever get to.
That case has good reviews and plenty of space to make building easy. If you want to reduce noise you could replace the tri-cool fans that come with it, but it's not necessary.
Those 480s are expensive.. I don't have time right now to check benchmarks, but would dual 570s or even HD 6950s give better performance for less money?