With 2 gig of ram, you will not need to worry about running out of memory. Task Killers are useless, if you set it to aggressive, it will keep on turning off apps and programs that needs to run in the background for the operating system to run smoothly. By having the phone keep on having to turn those programs back on in order for it to run properly is actually counter productive and will kill more battery than it's trying to save.
I just hold down the home button and it pulls up all the previous apps I have used. You can kill them all from there without having to worry about killing the ones the system needs.
There's no need to kill apps. The Android operating system is very good at managing system memory. Having apps in memory does not affect your battery life or performance.
It does if they are running in the background. If you dont use them you can clear data and disable to clear up memory. Just an option...If you dont want to do it or dont think you need to, then don't.