Unfortunately, most people coming to Android are used the the Windows OP and the way it handles memory. Android is based off of Linux and handles memory in a completely different way than Windows does. Get out of that 'More free memory = faster response' mode because that isn't the way Android is set up.
Killing off apps that are open does nothing to
free up memory because Android is designed to
ALWAYS keep things open in memory. It keeps everything you're doing with your device open in memory, and assigns those tasks with a priority based on what you use the most. So when you're doing something that's memory intensive, it closes out the app(s) with the lowest priority automatically to make room for the new activity. It does all of this for you, you don't have to think about it at all, it's automatic.
That's the whole point, the reason Android devices work the way they do, because the OS keeps everything open in memory. This way when you're browsing around on your internet and then you go to your music player to look for a song to listen to, it doesn't close out your browser session, it's still there waiting for you, so when you switch back, BAM, you're right back where you left off. No need to wait for your phone to open your browser back up again and all that.
I know it's a little hard to get used to when you make the switch, it's probably the number one most misunderstood thing about Android. Everyone coming to Android for the first time jumps on here and asks, "how do I free up memory"? But Android is designed to use all of your memory up, it only keeps like a 50 mb buffer to allow for other programs to start. But no matter how many apps you kill off, it's always going to saturate all of it's memory up to that 50 mb buffer.
If you want a larger buffer (like many of us do), once you root your phone you can download this app...
AutoKiller Memory Optimizer - Android app on AppBrain
With this app you can set your own buffer zone. I have mine set to 'Aggressive', which moves my buffer zone up to around 90 mb.
There's much to learn. It's really cool though so dive in. But forget app killers and all that jazz, having all of your memory used up is the way things are set up to work here at Android. Learn to understand that concept.