I'm guessing you mean 40Mb real memory and that you notice your machine slowing down (which does seem to happen once you get below 40-50Mb). The VM system allows you to have have more than 2Gb of memory being used by applications, but I do see noticable slowdown when I have iTunes running and a lot of active browser tabs - i.e. a lot of active memory.
My guess is that the real culprits are iTunes (if you have a large collection) and Chrome (15 tabs can eat a lot of memory if the pages are not simple text display). Also all the web browsers seem to slowly leak memory - periodically doing a real 'quit' can recover a lot of active/wired memory.