Check out the Operations Manager 2007 R2 Sizing Helper.
The Operations Manager 2007 R2 Sizing Helper is an interactive Excel document designed to assist you with planning & sizing deployments of Operations Manager 2007 R2. It helps you plan the correct amount of infrastructure needed for a new OpsMgr R2 deployment, removing the uncertainties in making IT hardware purchases and optimizes cost. A typical recommendation will include the recommended hardware specification for each server role, topology diagram and storage requirement. The Operations Manager Sizing Helper is most useful when used with the Operations Manager 2007 R2 Design Guide (http://go.microsoft.com/fwlink/?LinkId=184377).