Sizing for the SharePoint Web farm is challenging portion for SharePoint Architects  in designing a large web farms.

Finally HP has given a tool to size your web farm. This tool is available for two versions of SharePoint 2007 and 2010. 

Yes this is really good tool which give almost equal configuration when we manually size the servers.

 Articles

Capacity Planning Docs: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=fd1eac86-ad47-4865-9378-80040d08ac55&displaylang=en 

http://sharepoint-sandbox.com/index.php?/Practices/Recommendations/how-to-determine-farm-throughput-requirements.html 

Tools :http://h71019.www7.hp.com/activeanswers/Secure/548230-0-0-0-121.html 

Thanks
E.Ravi