Let your OS determnine this as "Ram" in a windows machine gets tied up and not released until you do a "Hard" reboot...you can get into all kinds of trouble messing with it...unless your comfortable with trouble
....or running Linux/Unix or a Mac which handle memory better...As was pointed out above...different programs can hang onto ram and you find your self in a pickle.
If you need more ram...you need more ram...or a better computer.