Actually, you can use just the upgrade version and fully install Vista without having any prior copy of Windows.
You simply install it once, but skip entering the product code on the first run. You get the full version of Vista, but in trial mode (for 30 days). Then once it's running, you run the install program again from within Windows. The upgrade version will detect the existing Vista OS and will install again, and this time you enter the product code. Then the copy of Vista will activate without a problem and voila, you've got a full version of Vista for the price of the ugprade software.
That's how I'm running Vista Home Premium via Bootcamp on my 24" iMac.