Today Microsoft made Windows 7 available to their MSDN and TechNet service subscribers, and to help prepare you for the impending release, the company has also produced an official upgrade chart.
(Click the image above for a closer look.)
According to ZDNet, "the blue boxes are for custom (clean) installs; the green boxes mean you can do an in-place upgrade". Granted, a chart's better than nothing, but folks (like ZDNet's Ed Boot) are also calling it out for being altogether confusing; in fact, Boot created a simplified version (click through to ZDNet to see it) that cuts out whole rows and gets straight to the point. Take a look at one or both for a closer look at what your upgrade path will entail.