i double that, Vista uses ram because it can, why buy 1GB of ram and have your OS only just 200 or so MB of it(XP). i idle at roughly 30% of my 2GB, which is 600MB, thats with Aero enabled too, turn Aero off, and let windows adjust to having just 512MB of ram and i bet it could idle at roughly 350-450MB easy.
last night i was in the cryengine editor for Crysis, and that thing was eating up my ram, and windows automatically started to lessen the RAM it was using, Vista went from using 30% to 20% so the editor could use more, the editor performed fine aswell.