Win 8 vs Win 7

I am building a new pc tommorow and want to know which operating system I should go for. I do gaming, and light productivity work.  Also can CS:S, CS:GO run on win 8?  Which do u thnk i should get.

I would go for 8 because it's objectively faster by technical metrics and as far as gaming, Linus recently showed in his 8350 / 3570 videos that on Windows 8 minimum framerates were often better. Those games will run fine on Windows 8.

Modern UI / Start screen isn't nearly as bad as some people like to think it is, and you don't have to use any Modern UI apps if you don't want to. If you really want to stay with the dying Start menu paradigm, then you can always bring it back with 3rd-party apps.

Origin and steam work on win 8 right?

Yeah. Pretty much anything that works in 7 will work in 8, there are exceptions, but not many and even if something doesn't there's usually a workaround.