You don't need a task-killer, they can be very detrimental... if you are worried about battery life, try using Juice Defender Ultimate. been running it for over a year, and battery life has been outstanding since I installed it. As far as memory management goes - never had an issue with the way android handled memory. Everything I use has always been available when I wanted it, and it has always ran smoothly.
If you're having problems with memory, you may want to take a look at what you have installed on your device - an over abundance of apps all running in the background checking everything you can imagine will take a toll.