Ok, so im thinking i want to buy vista, ill spend the money on Windows Vista Home Premium, but if i want i can upgrade anytime to the ultimate? How does that work, a new serial to enable options or a new download?
AFAIK all versions are on the DVD that you receive. The key that you obtain originally will unlock the version you paid for. In the event you upgrade, you get a new key like Lord said, which will unlock a different version from your DVD.