We posted a link today on the Interop site to the Microsoft System Center Capacity Planner. This tool allows you to plan for rolling out an Exchange infrastructure. For more information about the tool, check out Jonathan Hardwick's blog. Jonathan is a software architect working on this tool.
Note: You need to be a TechNet subscriber to download this tool. (On a personal note, before I joined Microsoft, I had a TechNet subscription and found it extremely helpful - gave me copies of software to install and use and become familiar with, tech support, etc. at a very reasonable price. If you don't have one yet, it's worth checking into.)