I have discussed the /3GB and /PAE switches previously in this entry: http://spaces.msn.com/members/russkaufmann/Blog/cns!1pwuGkyvTDx37q1_Y3JQ_E6g!229.entry

There seems to be a problem with the Exchange Best Practice Analyzer in that it incorrectly recommends that some Exchange servers have the /3GB switch implemented.

Refer to http://support.microsoft.com/kb/815372/ 

The key in this article is the note: "For Exchange Server computers that do not contain any mailboxes or public folders (such as mail gateways), we do not recommend setting the /3GB switch in boot.ini, independently of installed physical memory size."

Also, in http://support.microsoft.com/kb/823440/ it says:

"When you install Exchange Server 2003 on a Microsoft Windows Server 2003-based computer that has 1 gigabyte (GB) or more of physical random access memory (RAM) installed, and that is home to mailboxes or public folders, you must edit the Boot.ini file to optimize the virtual memory usage of the Information Store (I added the bolding) service."

The store.exe utilizes and leverages the addition RAM available in user mode when you use the /3gb switch. However, front-end (OWA) and IMR servers (which do not have information stores) actually suffer when using the /3gb switch and they should not use the /3gb switch.