TechSport posted an article on Windows 7 upgrade paths
>> Understanding Windows 7 upgrade paths
Windows 7 has already earned itself a blue ribbon, and it isn't even in stores yet. Many Vista users will migrate to Redmond's latest and greatest operating system in search of greener pastures, and countless Windows XP laggards will be lured from the woodwork by Windows 7's polish.
Microsoft will be granting both XP and Vista users the right to buy "Upgrade" versions of Windows 7 which run for considerably less money. But in terms of how you can perform an upgrade on a machine already running Windows, for some the in-place upgrade option will make the experience seamless; all your programs, files and settings will remain untouched coming from certain versions of Vista. However, for another significant portion of users, installing from scratch will be the only route available (and a recommended one at that).
Microsoft has released a chart to help users figure out what upgrade paths are open to them. Unfortunately, the chart is so cluttered and dysfunctional that it makes the whole process seem much more complicated than it really is. In an attempt to smooth things out for our readers, we've created a condensed, easy-to-digest reference with all of the potential upgrade paths.
>> Understanding Windows 7 upgrade paths