A great tool is the HP sizer for Exchange 2010.
About the HP Sizer tool:
microsoft® Exchange server 2010 is a complex application that requires intensive knowledge of deployment strategies, interdependencies, server and storage performance related issues.HP has developed the HP Sizer for microsoft exchange server 2010 to assist customers with proper server and storage sizing for their Exchange Server 2010 deployments. The algorithms developed and implemented in this tool are based upon extensive testing of Exchange Server 2010 server roles and technologies, HP's deep experience with Microsoft Exchange, and intricate knowledge of server CPU, memory, and I/O subsystems. This tool is free to download and use, and is designed to simplify configuration across a broad range of deployments. This newest Microsoft Exchange Server Sizer provides support for multiple site deployments, varied client types and access methods, all Exchange Server 2010 server roles, Database Availability group (DAG) high availability options, choice of tower, rack mounted, or HP BladeSystem server platforms, and either direct attached or SAN-based storage solutions. Based on your input, the tool provides a comprehensive bill of materials along with a deployment overview of the Exchange Server 2010 server roles and storage configurations. Once this Sizer is installed, updates can be downloaded automatically to the underlying software engines that are necessary to provide support for HP server and storage product information details.
Please see the ‘What's new” link on the home page of the Sizer (when installed on your system) for the most recent information.