One of my old colleagues J.D. Meier has written a performance testing guide with the Patterns and Practices team.

Here is a link to the guide, it's well worth reading:

http://blogs.msdn.com/jmeier/archive/2007/10/27/now-on-msdn-patterns-practices-performance-testing-guidance-for-web-applications.aspx

Laters,
Tess