Usage of /3GB and /PAE flag for various applications
The default setting should not include the /3GB switch. Microsoft thinks the best solution would be a matrix of physical memory, OS version, and application.
For Active Directory(Domain Controllers)
Always use /3GB for Active Directory
For Exchange servers
Always use /3GB for Mailbox and Public Folder servers
Never use for SMTP Bridgehead servers and Front End servers.
For SQL Server
In general /3GB is a good thing for SQL server. In some scenarios you have to examination such as multiple instance clusters, and high memory (12GB+) systems.
For File and Print Servers
Never use /3GB on file/print servers or terminal servers
For Terminal Server
Never use /3GB unless you need to run extremely huge application
Do not use /3GB across the enterprise as a default setting. You have to analysis application / services to determine if /3GB switch require or not