This is an article on Technet I want to share with you! I use it very often to make technical designs for clients:
Plan for software boundaries (Office SharePoint Server)
This article provides information to help you understand the tested performance and capacity limits of Microsoft Office SharePoint Server 2007, provides information about the test environment and test results, and offers guidelines for acceptable performance. Use the information in this article to determine whether your planned deployment falls within acceptable performance and capacity limits.