I found this wonderful article describes the optimization of memory usage on your computer that is running Exchange Server 2003.
For example, if you have 1 gigabyte (GB) or more of physical memory (RAM) installed on a server that is running Microsoft Exchange Server 2003, you must make sure that Exchange 2003 can make efficient use of that memory.
Please Note: Exchange 2003 performs an optimal memory configuration check when the store process starts. If the memory settings are not optimal, you will see event 9665 in Event Viewer. This message appears if one or more of the following conditions are true, as specified in the MSDN article.
It has many recommendations for each of the suggested settings.