In Windows Vista, Microsoft is making an effort to help you figure out where your weaknesses are when it comes to PC performance. The Windows Experience Index is an evaluation of key components of your system. When you want to know what to do to improve performace, you can take a look at it and decide what areas to concentrate on.
I think this will be a good tool for companies looking to move to Vista from XP. You can test your various machine types and look at the Experience Index to determine if you want to replace the machines, add memory, or just move to Vista as is.
Here is a related article: http://blogs.zdnet.com/hardware/?p=86