For aiding people in sizing their Exchange 2010 mailbox server configuration you can use the Mailbox Role Calculator. As of version 12.3 you can use the Intel SPECint2006 rate value to make calculations regarding the CPU. Unfortunately, not everyone owns a specific configuration shown on the SPECint2006 list.
The Exchange Product Group has come up with a tool (XLS) that calculates the average rate value for a specific CPU, using online SPECint information of configurations with the same CPU. Although not exact, it should provide you with guidance on the megacycles per core value to use in the Mailbox Role Calculator.
You can download the Exchange Processor Query Tool here.