Available Memory in perfmon

On a Windows 2000 server machine with 2 CPUs and 4GB of RAM, in idle state,
perfmon shows Available MB in the Memory Object to be around 3500. As users
login, this drops (proportionally to the number of users) to around 1000MB
(950-1050, say) and then stays there, regardles of how many more users login.
Is there a setting for this lower limit of available MB? It appears to
trigger a higher %PageFileUsage, which I assume means more "stuff" gets sent
to the paging file to obtain this 1000MB of Available memory. Can one lower
htis limit? More importantly, should one?
Any answer, or even reference to the literature would be greatly appreciated.

Thanks,
florind