You must have a lot of background processes running all the time. I have 6GB ram and with a video editor, graphic editor (lots of stuff open in it), 9 browser windows, music player all running right now it's only at 1.5gb of ram being used. If I open & run FNV on top of all that, it goes up to about 2.5-3GB.
Very few, if I check now, Windows reports 243MB free but with 2.5GB of cached data, essentially that's still 3GB of free RAM with a few browser windows, uTorrent, VLC and MSN messenger loaded.
The advertised upside is that Windows is pre-loading your regularly used data, the impressive thing is that it actually seems to work.