I see you are having trouble figuring out why your PC or laptop does not use all of its memory. First I want to tell you that you do want to use at least 30-35% of your memory, however try not to go overboard by using 80% and above for it could be harmful to the components. You said at time you get as low as 256 mb memory put in use, which makes me assume that you use either a Linux or Windows XP or below, simply because Vista is set up and unable to function if it doesn't have at least 1gb or ram. Trust me RAM is needed even though it is not always used to its full capacities. You always want to make sure you have that extra piece of mind in case something goes wrong e.g. your performance needed outreaches performance applied, so usage of RAM increases tremendously.
Another reason you might want to keep your memory at least 4gb is because a 64-bit Vista or 7 won't even agree to install on anything less. And I personally prefer a 64bit OS.
Hope it helps.