Right now I have Firefox, gpu-z, azureus, fraps, logitech game software (background), wmp 11, and antivirus (background) all running and im using .97gb with vista home premium!!
those aren't ram demanding applications except for vista...
i know that crysis alone can take more than 1gb memory for instance (with mine it does).
And like i said before, sometimes i've got 4 virtual machines running (512mb dedicated memory each) eats away 2gb completely.