That's not really how memory works in Android. The operating system will kill off apps as needed based on what they're doing and how much memory is free. If the system doesn't believe you're low on memory, it will keep apps in memory (or keep only their startup activity in memory) so that they don't have to load from flash when you open them again. Automatic task killers interfere with this process, and so really don't help with performance. Instead, you'll get processes getting killed off, then starting up again because some other task is needing them, e.g. There's a FAQ here
What I use, actually, is MinFreeManager
. It requires root, but instead of killing off processes arbitrarily, it lets you tweak the parameters of Android's built-in memory management.