Is the AMD Opteron 6172 a good CPU? For 12 cores and $30 price tag it's literally dirt cheap. The only thing that can be a problem are the overpriced motherboards.
Its a low clocked Phenom II with more cores. Would be pretty good if you wanted a super cheap dual or quad socket system for rendering video, but if you plan on using it as a desktop its not going to do much for you. Buy a cheap quad core Phenom II or an older generation APU.
If you want to use windows, then windows will ask for a license per cpu iirc, not up to date with their licensing model atm though. I don't think 2 or more cpus even work on the regular windows, so you need windows server 2012 or something.
I recently installed Windows 10 on a DELL precision 490 with two xeons and I can say that it works fine
Anyone who has had the pleasure of working with the quad socket g34 boards would not recommend them to anyone. Not only do they get shit single thread performance, but they also perform like shit in large multi-threaded tasks because of how AMD set up the caching on them. The motherboards are finicky and unreliable, the processors are slow for most applications, and the box will be serving a dual purpose as space heater while under load.