I've upgraded my *cough* pirated *cough* copy of windows 7 ultimate 64 bit to windows 8 (not pirated) and it went smoothly.
I'm using an old Mac Mini (early 2009) and it's working great. Just go the upgrade route, it transfers all your files, settings and drivers painlessly. It's much less of a pain in the ass than a clean install of windows and it works just as good.