I see that Newegg is selling 'system builder' copies of vista for much, much cheaper than retail. I'm just curious as to what a 'system builder' license even is, I can't seem to find a really clear, concise definition of what it is.

You mean OEM? It basically means that you can't install Vista on another motherboard than the one the first install is done on, although Microsoft isn't really enforcing that, just asking if it's installed on another computer and giving you a new code.