/3GB Switch on DCs
On Win2000 a 1GB DC will benefit from the /3GB switch. The benefit would start when the DIT and RAM are both over 0.5GB and reach a maximum when the DIT is 1GB and RAM is greater than ~1.3GB. Beyond that things don't get worse, they just don't get better.
On Win2003, the benefit doesn't really start until both DIT and RAM are greater than ~1.5GB, and you reach full benefit by the time both reach ~3GB.
If the DC isn't serving some other role than needs lots of kernel space (e.g., a fileserver) then there's an upside for all but the smallest configurations and essentially no cost.