I was working for one of the clients on the Exchange 2010 SP1 migration solutions, i realized, there is new version of Exchange 2010 mailbox server role calculator has been released, which i was not aware of & thought of posting for others, if someone is not aware like me,refer the below link with nice explanation of enhancements & improvements over previous versions.
It does give decent details of IO, memory,space etc. and it is targeted for exchange 2010.
http://blogs.technet.com/b/exchange/archive/2010/01/22/3409223.aspx
Download Exchange 2010 Mailbox Server Role Requirements Calculator v18.2 from the below link.
http://gallery.technet.microsoft.com/v144-of-the-Exchange-2010-1912958d
