Why do you want it to use swap? Swap is super slow, avoid it at all costs. The kernel is smart with memory management, it doesn't free memory once you're done with an app, it keeps it in a cache so next time you use that app it's all there in memory waiting for you to go. If something else needs it then it'll make that remove the cache and make the memory available to other apps so in reality, linux is very good at looking after your RAM and in your case, 1009MB used - 621MB in cache = 388MB used really.
At the moment I'm running X, fvwm, firefox, gaim, gmpc and a few ncurses apps and I'm using 196MB so running KDE thats probably not too bad.