I've been using Windows since 95 and I can tell you, Windows 8 isn't as bad as reviewers say. The boot up is 2x faster than Windows 7, looks nice and clean and does the job.
Windows 7 is still a great OS for Windows, nice and stable and does the job. But if you fancy exploring, check out Windows 8.
And yeah, build your own computer, its the best thing to do. To get really nice FPS, you don't need to get an i7 with 16GB's RAM etc etc. Those are like the nice extra things that you don't really need. For example, I have an i5 3rd gen and it does a wonderful job with my 8GB RAM. I'm currently play Planetside 2, which likes to eat up CPU and GPU, and I can play it on max settings and still get the recommended 60FPS (without overclocking). I could get more if I had a different GPU (I have the 660, could have got the 700's instead) or if I bought the 4th gen i5