I have done something like this in the past (not for gaming, for software development). The author basically bought an older server based system, and Intel CPU’s, and build a rough gaming box that preforms pretty well for a system that I think was 8 years old. My 2 cents, I have done this in the past myself for “software development” systems where I need the horsepower for compiling and such.
http://lifehacker.com/build-a-super-cheap-gaming-pc-with-old-server-parts-1747549635