I'm not sure why you must have cas 7, cas 9 1600 would be faster than cas 7 1333 although it makes close to no difference. http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/6
You would also be better off with ram at 1.5v. So get 2x4gb, 1600, cas 9, 1.5v. Of course the lower the cas and faster the speed is better if you can find them at similar price.
You won't find 6gb kits as there are no 3gb sticks. The triple channel kits are not for SB.