There is nothing better to avoid headaches then a good planning for the technology that you are about to implement. Many scenarios of poor performance or inappropriate behavior sometimes happen because administrators skip the planning phase and go direct to the implementing phase. My advice here is: try to avoid this rush as much as you can, skipping planning phase can cost you more than you think. You just need to imagine the downtime that you might have when the environment is already in production and your server is not capable to handle the amount of requests that you have. Think about it!
From the Forefront Security for Exchange Server perspective we have good news, recently released the Forefront Security for Exchange Server SP1 capacity plan tool. This tool will assist you during the planning phase to correctly size the hardware that is necessary to run this product, among of many other things. Got get it at: http://www.microsoft.com/downloads/details.aspx?FamilyID=522da65d-5263-4f5d-b929-8428a394b9af&displaylang=en
Happy FSE Planning Phase for ya!!