The Microsoft documentation for Exchange 2000 Server suggests minimizing the number of storage groups (SGs) to reduce resource overhead. However, the Exchange Server 2003 deployment guide doesn't make any mention of this guideline. Why is that?

The guidance changed. Each Exchange 2000 SG has a separate instance of the Extensible Storage Engine (ESE), so adding an SG in Exchange 2000 imposes a memory-consumption penalty of about 100MB, even when the SG is empty. Exchange 2003's SG allocation and management processes are significantly different from Exchange 2000's. Exchange 2003 sees little difference in overhead between having, say, four databases in one SG or splitting the same number of databases between two SGs. Because the databases in an SG share transaction logs, you might find that creating additional SGs is a better alternative for your organization than creating additional databases. Take a look at the number of disks you have available and your database usage patterns before deciding which approach to use.