The system you're looking at there would tear up any games currently out and likely any still on the drawing boards. If money is not a factor, there are improvements that can be made such as a Corsair H100i instead of the H80 to give you lower temps/better overclocking ability and for extreme, two GTX690's in SLI.
There should be no problems running windows 7 professional or ultimate but not the lessor versions of windows 7 which do not support more than 16GB of ram
I think a case is a personal preference thing but my current personal preference is toward the NZXT Switch 810.
If you're going all out on a build like this, stay with the 3930k and get a Noctua NH-D14.
I would go for something like dual slot 670 TOPS, which would perform better than stock Gigabyte 680's. Gigabytes 670/680's are voltage locked, while the 670 TOP is near silent, not voltage locked, clocked extremely high, and is perfect if you decide to quad SLI later.