And for the OS, I would get Windows 7, unless you want 8.1 (that extra 0.1 is very important...).
Unless you want to try Linux. It is free, you could just try it. Then you could use that cash to get a better graphics card and/or CPU. I just started using Linux, and I must say, I will never use Windows again! (I don't play games, though. You can play most games in Linux, see this.)
i know its a little over $650 but you really dont want a processor less than the fx6300 and if you want to be able to have a very goo experience a 1080p in all games you dont want any less than the r9-270 video card.