Here is a list of links related to capacity planning and

Plan for Performance and Capacity (Office SharePoint Server)

Design the Logical Architecture

Determine Hardware and Software Requirements (Office SharePoint Server)

Tools for Performance and Capacity Planning (Office SharePoint Server)

Visual Studio 2005 Team Test Edition: Testing Demos

SCCP RC Download

SCCP Home Page

SharePoint Capacity Planning Tool (TechNet)

SharePoint Capacity Planning Tool (Beta)