Reasoning: Very nice case. Plenty of room for airflow and all of your components. Great cable management, window on the side, plenty of places for additional fans. Worth the money imo!
Optical Drive: Any cheap optical drive will do!
Reasoning: I'm using an external one because all of my drive bays are used up, but you can buy any cheap one to throw in your case for $10-$20.
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit)------------------$99.99
Reasoning: Memory limit of 16 GB, but you don't need that much or any more than 8 GB for gaming.
Total cost = $1,279.91
Now you can always make this cheaper by downgrading the case, maybe grab a 7870 or 7950 GPU, maybe get 2 gigs less of ram, but this is a very solid gaming rig IMO, and you can also add lots of fans because it is such a nice case!
If you have any questions feel free to ask, and make sure to tell me if I missed anything!