While designing a new exchange organization it is really tough to do the proper capacity planning. Most of the times it is really hard to decide how to choose correct hardware configurations. HP has developed a tool for exchange 2010 sizing problems. I found this tool really cool to be used as a help for your capacity planning and related stuff.
You can download this tool from HP’s website directly. Here is the download location: HP Sizer for Microsoft Exchange Server 2010
To read the primary documentation and introduction of the tool you can also logon to http://h71019.www7.hp.com/ActiveAnswers/us/en/sizers/microsoft-exchange-server-2010.html