We're upgrading our Exchange Server hardware in advance of our migration to Exchange Server 2003. Each of our servers can have up to 4GB of RAM. Can Exchange Server 5.5 take advantage of the extra RAM?
Exchange 5.5 was the first Exchange version to incorporate Dynamic Buffer Allocation (DBA). DBA allows Exchange 5.5 to grab most available memory, releasing it as other applications raise the system page-fault rate above a specified threshold, then taking it back when those applications are finished. A similar process is used to allocate RAM in Exchange 2003 and Exchange 2000 Server. So, yes, the Exchange 5.5 Information Store (IS) can directly use up to 3GB of RAM without any special effort on your part. In the configuration you describe, you don't even need to use the /3GB switch with Windows Server 2003 or Windows 2000 Server.