Microsoft published a new Exchange 2010 whitepaper titled "White Paper: Understanding the Relative Costs of Client Access Server Workloads In Exchange Server 2010." Here's what it's about:
"Estimating your Exchange Server 2010 Client Access server capacity needs is a critical setup task. CA server is the entry point for all users. In addition, CA server hosts important services used by other Exchange server roles. This white paper presents an estimate of the relative CPU weights of different protocols on CA server that can be used to produce a more detailed estimate of hardware needs when you design a new Exchange 2010 deployment or expand an existing one. As part of the testing performed while researching this white paper, the effect of MailTips and the cost of NTML versus Basic authentication were also compared."
More Info: Whitepaper