Easy vista question I suppose

[maniacut]

I've tried some google searching for my answer but haven't found what I'm looking for. Basically I'm selling Windows Vista Business Express Upgrade (OEM) and from what I understand to install this, you would need a previous MSFT OS installed like win xp for example. I was wondering if you could install the upgrade on multiple systems or is there a feature that prevents this from happening, i.e. can only be installed once specific to the motherboard of first install?