windows 8 is actually faster then 7 in most to all games
dont know why your saying go windows 7 for the performance.
In most tests, there's actually been little to no difference in performance between 7 and 8 on gaming PCs. There's only a real difference in performance when you have fairly limited hardware. Windows 8 runtimes use maybe 200-300 MB less RAM than Windows 7 runtimes (or they should, at any rate), but on a gaming PC with 8+ GB of RAM, that difference is pretty insignificant and will not affect gaming performance at all.
This may change when/if games start taking advantage of DirectX 11.1 and 11.2 features, but for now, the two versions of windows are functionally equivalent, or whatever minor differences they have are barely noticeable. Really, it comes down to a matter of aesthetics and potential "future-proofing." If you want to keep playing newly-released games several years from now without upgrading to a new version of Windows, get Windows 8 now and you're good for a long while. If you absolutely hate the look of Windows 8, get Windows 7.
Definitely take a look at Windows 8.1 before you decide though. Windows 8 had a lot of shortcomings, and the free 8.1 update will fix many (but probably not all) of them when it comes out next month.