From Windows Vista Secrets 2nd edition Chapter 3 (Hardware and Software Compatibility):

To help you determine whether your current PC has the performance characteristics and hardware and software compatibility needed to avoid issues before upgrading or migrating to Windows Vista, Microsoft provides a handy tool called the Windows Vista Upgrade Advisor.

The Upgrade Advisor performs an analysis of your PC, and is partly designed as a marketing tool, as it will "recommend" which version of Vista is right for your system. (Humorously, it almost always recommends one of the more expensive, premium versions.) But the Upgrade Advisor also provides real-world benefit outside of Microsoft's needs: It will tell you which hardware devices and software applications will need updates before they can work with Vista. And because the backend of the Upgrade Advisor application runs up on Microsoft's servers, it always provides up-to-date information.

While the Windows Vista Upgrade Advisor is primarily designed to help users of previous Windows versions discover whether their PC can be upgraded successfully to Windows Vista, it also has a secret second use: It can also be run on Windows Vista and used to determine whether your PC is capable of running a more capable and expensive version of Vista.

The Windows Vista Upgrade Advisor can be downloaded from the Microsoft Web site.