Great, great post on the Microsoft Forefront Server Protection blog: Forefront Protection 2010 for Exchange Server (FPE) Capacity Planning Guidance v. 2.
“The purpose of this document is to provide guidance into Forefront Protection 2010 for Exchange Server (FPE) capacity planning regarding hardware, such as the number of processing cores, memory requirements, and when to scale up based on CPU utilization and throughput. Performance guidelines using the capacity planning tool are provided for Exchange Server 2010 deployments. A new interactive capacity planning tool will be released in the future for FPE 2010.
Performance guidelines are also provided here on how to leverage the existing Forefront Security for Exchange Server (FSE) Version 10 with Service Pack 1 capacity planning tool for Exchange Server 2007.
Note: The performance measurement baseline information provided is based on the existing product documentation for Exchange Server.
All organizations are unique and driven by requirement specifications, policies, behaviors, and cultures that guide and inform hardware purchase decisions. This document can help clarify the additional load created by FPE for Exchange Server 2010 on sample server environments and provides guidance to help with capacity planning decisions. This information should be combined with your experience, the Exchange server capacity planning guidelines, and your general knowledge of your own organization and IT landscape.”