A. Microsoft System Center Capacity Planner 2007 is a great pre-deployment capacity planning and post-deployment change analysis tool that gives hardware recommendations and utilization metrics for specified environments (e.g. designated physical locations and numbers and types of users).
If you want more general Exchange 2007 memory- and processor-sizing information, "Exchange Server 2007 Processor and Memory Recommendations" is a great resource.
The article gives advice on the minimum and maximum number of cores for each role. For example, it recommends that a Mailbox server have a maximum of eight cores for the best price and performance ratio. It generally recommends one core for every 1000 moderate users, 2GB of memory for processing, and 3.5MB of RAM for each mailbox. So, for example, a server with 4000 mailboxes should have 16GB of memory. The article also recommends the maximum number of supported storage groups (SGs) for designated amounts of memory. Using the earlier 16GB of RAM, it recommends a maximum of 32 SGs.