this is why you do not need a task killer. the OS knows how much memory it needs to run the apps you already have. this is a perfect example.
now you may be able to "fool" the OS by force closing some apps that are sitting in RAM. this will increase your "bucket" of memory which may let you install an app, but remember if you do these types of things you will only cause issues down the road.. lagg, error messages, etc.(because you are fooling the OS in thinking you have given it additional memory which in fact you did.. you only force closed).
best to either uninstall an app or two you dont really use.. im sure you have one or two that gets used RARELY if at all.. ROOT and then you can move apps to SD.. or wait for 2.2....