Hardware requirements and recommendations for the Windows Vista Capable PCs have been selected to ensure that PC systems and components that fit these criteria will run Windows Vista and will enable the next generation of Windows applications. Enterprise Planning Guidelines are based on these hardware requirements and recommendations. By using these guidelines to procure PC system hardware, customers should be able to deploy systems that will upgrade well to Windows Vista after the launch of Windows Vista. In this way, customers can protect their PC investments today."
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An excellent read that provides some improved clarity on hardware support for Windows Vista. Also check out the link to a list of ‘modern CPUs’ that support Windows Vista. This has not been very clear to be honest, but I would say its finally begining to show.