…users of XP Home can do an upgrade install to any of the four Vista versions. However, XP Pro users can only perform upgrade installs to Business or Ultimate. Windows 2000 and Windows XP Pro x64 users can do no upgrade installs at all, and must perform clean installs (either on a fresh hard drive or an empty hard drive partition) for any Vista version.
If you are planning to upgrade to Vista, here is all you need to know. It’s interesting that XP Home offers four upgrade options, while XP Pro only offers two.