SBS 2000 mem usage

I have a sbs 2000 running on w2k server, the server is a IBM e220 1Ghz cpu with 896 mb ram, just put in another 512 ( before 384 ), after approx 24 hours there is only 5% mem free, we are 4 workstations in network, My question are, do I need even more mem than 896? Of course this depends on usage of server, but what I can see och cpu usage on server ( normaly between 2-30% ) depending on what we do. Before when with 384 mb ram had about 2-5% free mem, and now same thing happends but now I have 896 mb, is this still not enought mem?