Microsoft has updated the Mailbox Server Role Requirements Calculator for Exchange 2010 yet again and the latest version is 7.7
Download it here
Exchange 2007 calculator has been updated as well and can be downloaded from here
The Exchange 2010 Mailbox Role Requirements Calculator has been updated again, the latest version being 4.5. I would urge all readers to keep an eye on this space, as new versions will be uploaded. The latest version has included new options like considering whether you have a 32 or 64bit GC in the environment. The calculator only supports active/passive environment even now.