Best windows for gaming?

Solution
That is almost completely a matter of preference I have heard that there are some advantages to windows 8 (I believe Directx 11.2 only works with windows 8 for now) as well as some times being able to more fully utilize multi-core processors.

I still prefer windows 7 and use that, however I have heard there are some advantages.
Wait, where did you see free upgrade option for Windows 7 to 10? That would be so cool...

I have heard rumours of Windows 8 to 10, but not 7.

Anyway, both work equally well for gaming, its all about preference.

EDIT : Some things to consider on your build.

PSU : Complete overkill, 550W is more than enough. Unless you plan to SLI sometime of course.
CPU : I5 is just as capable for gaming, but the I7 does have a bit extra recently when it comes to SOME games, although its rare.
Cooler : MILD overclocking is good on that cooler. For more, get a Noctua NH-D14.
Storage : At your budget an SSD would be great.


 
That is almost completely a matter of preference I have heard that there are some advantages to windows 8 (I believe Directx 11.2 only works with windows 8 for now) as well as some times being able to more fully utilize multi-core processors.

I still prefer windows 7 and use that, however I have heard there are some advantages.
 
Solution
I would wait until win 10 has got more reviews, then you can decide if it it worth going for 7 or 8.1 as 8.1 does allow you to go to the desktop on boot which will hide the live tiles which is the only thing different about the 2( in appearance) and the start button is also different.