i use "advance task killer" and when you open it, you can check what ones you want and don't want to be killed and then on the top of app there is a button that says kill, and it says how much memory is being used by them running. emails, texts, call, they all still come through. if i use a couple things or a game, i always use task killer after it cuz games if you keep them running can slow down your phone.
is your task killer icon red? mine is green. if you go to market and search advance task killer, or probably just task killer, it will show up. it's the first option when i do it.