Preventing Akonadi/Nepomuk/Strigi nonsense from running helps with resources quite a bit (unless, of course, one actually uses them). My Ubuntu with KDE uses less than 400MB RAM idle and without the above.
That includes kwin, window manager
plasma-desktop, desktop and panel
krunner, application launcher
kded4, multiple daemons
A complete session of KDE includes Nepomuk and Akonadi which are libraries helping KDE's applications, not the 'Desktop Enviroment'.
Disclaimer: Im a KDE contributor.
Edit: formatting.