Samsung SyncMaster 943SNX 18.5", DVI to VGA Conventer...
The SyncMaster 943SNX monitor definitely has a built-in power management system called PowerSaver. It should switch your monitor to low-power mode when it has not been used for a certain amount of time.
Was the DVI to VGA/D-sub adapter, that you are using, included with the graphics card or did you obtain it separately? If it wasn't included with the graphics card then that may be the cause of your problem. DVI to VGA/D-sub adapters are not all created equal.