I would get the AMD FX-6300 for the CPU.
The GTX 750ti is a great low-budget gaming card.
Get a high-quality power supply, at least 400-500 watts.
For storage, I would get a
Samsung 840 Evo.
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.)