Planning for Capacity Boundaries, Estimating Performance & Capacity Requirements, Additional Factors, and Tools
WSS: http://technet2.microsoft.com/WindowsServer/en/library/0e13e879-0156-4e9f-b6d5-abddcf326d211033.mspx?mfr=true
MOSS: http://technet2.microsoft.com/Office/en-us/library/eb2493e8-e498-462a-ab5d-1b779529dc471033.mspx?mfr=true