Very nice guide. But I wouldn't build a Steam Box. There's something to be said about consoles and how they provide developers with one standardized specification that they can target over a span of 6+ years that enables them to optimize games so they run smoothly while squeezing out every last drop of power the console has to offer. Gaming PCs lack this standardization that consoles enjoy. Sure, there's infinite flexibility with what you're able to do with a PC -- such as using it as a HTPC -- but over time, a Steam Box built today won't be able to play the latest games that 2017 has to offer with any grace, while the PS3 will be running a title like "The Last of Us" nearly seven years after the console's introduction. Sure, you could always upgrade the graphics card, CPU, HD, and other components, but now you've already overpaid what you would have had you bought a PS4/Xbox 720.
Just something to keep in mind for anyone who decides to build a Steam Box in hopes that is can replace a console.