Last week I wrote and article (!554AE8DB90904219!278.entry)  about SharePoint 2010 capacity planning.  It is important to have the right tools and to have metrics to use in order to plan a SharePoint 2010 infrastructure.
Microsoft has created a SharePoint 2010 capacity management site to centralize the documentation and tools to help you plan and size your SharePoint 2010 farm.  Here is the link: