Free ram is just that - unused... worthless.
It's not important how much ram is used by the OS it's important how much of that is "unswapable" (that's the point were the usual OS functionality gets to a crawling speed).
What you mean, I think, is "how much is in the working set". I.e., what is being used by the OS from minute to minute, as opposed to being e.g. an HTTPserver daemon that has no connections all day, but perhaps gets them at night when you're AFK.
Most OS's are rubbish at deciding what to swap to disk when. Most do pre-emptive swapping (NT4 used to try to swap something as soon as you hit the minimize button - i.e. if you liked to used minimize to navigate from window to window, you were ****ed by MS). Linux seems to just say "this app hasn't done anything in X minutes, I'll get rid of it" - which is equally stupid because if you go away from lunch, whne you get back ALL your apps are now swapped out to disk! It *is* a tricky problem, pre-emptive swapping
IIRC Windows tends to keep everything in RAM unless it runs out. That means that it is important how much memory the OS is using. At the very least, as soon as you load a big app you'll get a sudden slowdown as tonnes of irrelevant cr*p you weren't actually using gets swapped to disk to make way for the app. Rather like simple GCs
However, the great thing about windows is that your memory-resident-but-invisible apps (anyone recall TSR's
? ) are very easy to manage. Services panel lists all of them, with easy controls to remove/disable/start/stop them. Rather than faffing about with Enlightenment, it would be very nice if Linux devs copied the really simple parts of windows (like this one) that offer the biggest gains in usability and productivity...