I get asked this quite a bit by my education customers. How can I get a good estimate for sizing my Mailbox Server, Hub Transport, Client Access Server and Storage?
The latest and greatest Mailbox Server Role Requirements Calculator v3.2 has some nice new sizing capabilities such as the ability to help you size the CAS and Hub Transport server in addition to Mailbox Server and Storage needs. This is nice since you no longer have to calculate the Hub and CAS server needs manually using the core ratio of 3 cores of CAS for every 4 cores of Mailbox or 5 Cores of Mailbox for every 1 core of Hub. It also has about 11 other changes to help smooth out the calculator use.
I highlighted some of the Hub, CAS server sizing additions in the tool for estimating 5000 mailboxes:
You can grab version 3.2 here which just released on Friday.
For more in depth sizing and capacity planning of Exchange 2010, I recommend you run the JetStress 2010 for disk stress and LoadGen 2010 tools for Mailbox server sizing.