One of the things I worked on for Vista was the System Requirements.   These are widely quoted, but the canonical source is Knowledge Base Article 919183.

There are two sets of system requirements

  1. The minimum supported system requirements:  this is the bare minimum need to run any SKU of vista.   If the system doesn't meat these then its literally not supported, even if you can get it to run.  
  2. The minimum recommended system requirements: these are the requirements we recommend systems have for the basic and premium SKUs.  

There is another set of important requirements, these are V3.08 of the Windows Logo System Requirements.   From a system perspective, the systems requirements govern how OEM's build systems that receive the Windows Vista logo.   These requirements  go far beyond the simple minimum supported and minimum requirements.  For example, requirement SYSFUND-046 specifies the system and graphics features and performance necessary for Aero.  You can stay up to date with changes to the logo requirements by subscribing to the WHDC newsletter.