If u have 4GB of memory and change this option to 1024 ~1GB then every client reserve 1GB of memory - it should reserve.
You can change other parameters to make your playing more comfortable but less attractive:
parameters from ClippingRange section (like terrain, pawns etc.), from WinDrv.WindowsClient section (all Textures parameters starting from TextureDetailInterface - u can change only between low, high etc.) ..you can change more and more parameters from l2.ini to make your playing more comfortable on lower machines/sieges etc.