I've never actually researched this, or collected any cold hard evidence, but it has always seemed to me that Windows XP (and possibly other OSes) scales its resource usage to system availability. Meaning that it will use 20% of RAM, whether you have 512MB or 4GB available. (Maybe not so much use, more like set aside for itself.) My one and only piece of evidence for this is that when using a fat computer (dual Xeons, for example), processor-heavy tasks are noticeably faster, but everyday OS tasks (copy, paste, drag, context menus, etc) are not. There's probably an upper limit to how much it will use.
But again, I can't confirm any of this.