I just wanted to take a moment to remind everyone about the SharePoint Capacity Planning Tool.

Official description:  The SharePoint Capacity Planning Tool comprises models of both Microsoft Office SharePoint Server 2007 (MOSS) and Windows SharePoint Services 3.0 (WSS) that allow you to explore the necessary infrastructure for either deployment based on usage requirements. This tool uses the System Center Capacity Planner 2007 (SCCP) as an engine to provide for data collection, visualization, simulation, and report writing. The tool can be used in pre-sales and feasibility studies of a deployment project to give you a rough estimate of hardware requirements.

The tool is able to consider variations in usage, size of installation, networking technology, scale, and availability.

http://technet.microsoft.com/en-us/library/bb961988.aspx