Calculate Your Exchange Server

Calculate Your Server Size
http://technet.microsoft.com/en-us/library/bb124226(EXCHG.65).aspx

Baseline Performance for Mailbox Server (Using MAPI)
http://technet.microsoft.com/en-us/library/aa998153(EXCHG.65).aspx

Calculate Your Server Size
http://technet.microsoft.com/en-us/library/bb124226(EXCHG.65).aspx

How to Calculate Mailbox Server Disk Subsystem Requirements
http://technet.microsoft.com/en-us/library/bb123859(EXCHG.65).aspx