You are using more than 1 GB of memory on your domain controllers and want to enable the /3GB switch so that the LSASS process can use more memory.
Edit the boot.ini file on the domain controller to contain the /3GB switch:
[boot loader] timeout=30 default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS [operating systems] multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Windows Server 2003" /3GB
Restart the computer.
When computers are referred to as 32 or 64-bit computers that means they support memory addresses that are 32 or 64 bits long. This is the total available memory (virtual and real) that can be processed by the system. Since the days of Windows NT, Microsoft has split memory allocation in half by giving applications up to 2 GB and the Windows kernel 2 GB of memory to use (32 bits of address space = 2^32 = 4 GB). In many cases, administrators would rather allocate more memory to applications than to the kernel. For this reason, Microsoft developed the /3GB switch to allow applications to use up to 3 GB of memory, leaving the kernel with 1 GB.
The /3GB switch is supported only on Windows 2000 Advanced Server, Windows 2000 Datacenter Server, Windows Server 2003 Enterprise Edition, and Windows Server 2003 Data Center Edition, and should be used only if the computer has more than 1 GB of physical memory. For a good description of how LSASS uses memory, see MS KB 308356.
MS KB 99743 (Purpose of the BOOT.INI File in Windows 2000 or Windows NT), MS KB 291988 (A Description of the 4 GB RAM Tuning Feature and the Physical Address Extension Switch), and MS KB 308356 (Memory Usage By the Lsass.exe Process on Windows 2000-Based Domain Controllers)