When you are architecting a SharePoint Portal it is very import that you understand the boundaries and thresholds that will impact performance and maintenance of the environment. You should always consider the capacity planning while architecting any solution. Microsoft has well documented and provided lot of resources around Capacity planning and that information can be found in http://technet.microsoft.com/en-us/sharepoint/ff601870.aspx.
Apart from these resources, for SharePoint 2007 Microsoft provided Capacity planning tool which really helped many people in designing basic architectural diagram and configuration for the SharePoint portal implementation using MOSS 2007.
HP (Hewlett-Packard) also provides an excellent Sizing Tool which helps many architects in designing and tuning Farms for SharePoint 2010. Also provides an understanding on Pricing of the Server Farm, you can do lot of tuning.
Download HP Sizer for Microsoft SharePoint 2010
Download HP Sizer for Microsoft Office SharePoint Server 2007
Download HP Sizer for Microsoft Office SharePoint Server 2007
Cheers!!!
