A process running under Windows 2000 or Windows Server 2003 can access up to 2 GB of memory address space, real and virtual.
The more Active Directory database that can be loaded into memory, the more Active Directory performance improves.
If you have more than 512MB of RAM, but less than 3GB, only a maximum of 512MB of Active Directory will be loaded into memory.
If you use the /3GB boot.ini switch, more of the Active Directory database can remain in memory, up to 2.6GB.
NOTE: If you use the /PAE switch, don't add the /3GB switch.