Folk who do SCOM deployment will find this tool come to handy when planning. The Operations Manager 2007 R2 Sizing Helper is an interactive document designed to assist you with planning & sizing deployments of Operations Manager 2007 R2.
To download:- http://www.microsoft.com/downloads/en/details.aspx?familyid=b0e059e9-9f19-47b9-8b01-e864aebf210c&displaylang=en
Use this tool with SCOM Design Guide (Click here).