I was reading through technet today to review some of the new features of Exchange 2010 and came across an interesting little note about Sizing for Client Access Servers.  In Exchange 2007 it was recommended to size a server of 1GB per core on the Client Access Server.  Well, reading here I noticed that it states for a large organization that you should consider doing 2GB per core with a Max of 8GBs.  A typical installation is recommended to have 4GB of RAM.  However, based on your deployment you may need to add additional servers to your environment depending on just how large.  MS is also recommending 1:4 Core ratio for CAS to Mailbox.  That is for every four cores on a MB server you should have 1 core for your CAS.  For example, if you deploy eight Mailbox servers in an Active Directory site, and each Mailbox server contains four processor cores (for a total of 32 Mailbox server processor cores), you should deploy at least eight Client Access server processor cores. These could be deployed as two Client Access servers with four processor cores each, or four Client Access servers, with two processor cores each.

Another interesting aspect is that Clients will now connect to the Client Access Servers for all access, this includes Outlook 2007/2010 as well as Outlook Web App.  This works rather slick when using DAGs and moving a users mailbox.  Since the user is connected to the CAS they will not see an outage, but rather the CAS will point them to the new mailbox server on the backend.  Having considered that all clients will connect to the CAS it may be recommended for additional memory on the server itself, if you’ have a large environment or perhaps you happen to have a CAS and HUB installed on one box. 

Time will tell as Exchange 2010 gets out in the world.  Much of the information is based on RC1 and Beta so who knows, it will probably change.  I do recall reading an article some time ago that a CAS should have no less then 16GBs or RAM. 

MS does have a good table for helping on sizing an Exchange 2010 Client Access Server.  To be honest given the fact that Exchange 2010 is supported for virtualization how hard will it be to adjust settings?

Check out Recommended Performance Counters, it’s at the bottom of the page.