This comment is possibly the one that confused istrike:
But I still don't understand why 12GB is a necessity for this build.
The i7 can support 12GB of RAM at 1533 (close to 1600Mhz) AFAIK. Haven't seen any evidence otherwise. With that said, you won't see a difference between 12GB of DDR3 1333mhz and 6GB DDR3 1600Mhz in games. So go for more RAM.
But I still don't understand why 12GB is a necessity for this build.