I don't know. You also have the option, if you own Vista Ultimate then you can license Vista Home Premium for like $50 on another computer. I'm not sure how it works, but it must check to make sure you also have Vista Ultimate, either by checking your other computers on the network or putting in both license keys or something. Information on this, again, is in like the System Properties or somewhere like that.
I have Vista Ultimate (not installed, so I can't double check for where the info is) and if I ever start using it (someday I'm sure I will) and I want it on my other computers at home (kids' computer, my laptop) then I'm going for the Home Premium addon license.