So far I've assembled three PCs from scratch, and while I recognize that this guide is very simplistic and could do with a bit more advanced stuff, it would still have been very handy when I was starting.
So, here it is:
Take it as a starting point and leave here your tips to build an awesome game machine. ;)
I'll give the first tip: don't try to go for cheap on the PSU. I learned that the hard way.