Rui does it again, this is a sweet article, read it and weep!
Implementing a complete Exchange 2007 solution requires not only good planning, but also a good understanding of all the technical requirements and a good knowledge of the several Exchange Server roles.
Sizing an Exchange 2007 Organization is not for the faint of the heart, although I consider it much easier than doing it for the previous versions of Exchange Server.
A correctly sized infrastructure will improve the general reliability of the system and the levels of service (meaning happy users), rationalizing the use of hardware (and money).
This article does not intend to be a deep technical dive into the sizing process, instead it will give you some rules of thumb in order to ease the process of planning the different Exchange Server 2007 roles of your messaging infrastructure.