Not bad at all. Except that it's using _too little_ (unless you just booted up your N900). Linux tries to use all the RAM, because (as biggzy said), free RAM = wasted RAM. Linux will read all files (including applications) _through_ RAM, so that next time something accesses that file (or application) it's already in RAM. Saves time, and battery (no need to actually access flash/disk whatever). Same when you write to a file, the data will be in RAM too, so that when another application comes around to read the file it'll get the data from RAM. That's fast.

As soon as an application needs RAM for something for itself (e.g. its data structures) it'll get its RAM from some of the RAM used as above (aka 'buffer cache'), unless there's still 'free RAM' (if you just booted, remember), and if the file that was buffered this way is accessed again it'll have to be dragged from flash/disk. There's no speed penalty in this either, at least nothing to talk about.

It's all very efficient.