I'm not exactly sure, but if I make an exact copy of a HDD with a Vista OEM OS, won't that basically give me an unlimited amount of hard drives with vista installed depending on how many copys I make?
No, not really. Because Vista keeps tabs on system details, and will force you to call Microsoft for a new Key. It's a fairly painless 5 minute phone call (toll free in the US). But if you're cloning installations, Microsoft will figure it out pretty quick.