Idling at 30%-50% RAM with 4GB or even 8GB is absurd! It's much too high usage for idling. I suggest you go start > msconfig > startup and disable almost everything in there, except things like firewalls and antivirus. You can do the same for the services tab as well, just hide all the microsoft ones (unless you know what they do) and disable most of the rest. I would however suggest you disable microsoft ones like remote registry. That just allows viruses and hackers a door into your system. Unless you get help via remote logons, that service is useless.
But yes, if you add more memory, windows will take some.