Build a Kick-Ass $500 Gaming PC, Play Crysis at 40FPS! | Maximum PC
They assume you have your own OS license already, which is the case with most people who look into building their own computer. They also assume you're not going to overclock.
It's actually a pretty decent build they put together. The only contentious part is the case/power supply.. if you spend an extra $15-20 you can get a much better quality, but still basic case and much better quality PSU.
XI Wiki



