If you have upgraded to Snow Leopard and the computer started terribly (terribly!) To slow down, then it seems I know what your (and sometime my) problem is. And the problem is in coreaudiod. Coreaudiod is the API responsible for sound in OSX. It happens that the coreaudiod starts to occupy all the free RAM. In my case, from 4GB of RAM, 8-10MB remained free immediately after booting the system. The solution is simple - remove the plist at: System / Library / LaunchDaemons / com.apple.audio.coreaudiod.plist, reboot and voila - everything works fine :). It is strange that this bug in 10.6.1 is not solved.
UPD. Let's not breed holivar from scratch. This is just a special case, I thought that the solution I found can help someone. This situation is not so easy to recreate and probably it is not entirely correct on its basis to build conclusions about any flaws in OS X.