I'm running Arch Linux with KDE 4.4.1, and I can start KDE using justt 260 MB of memory. With a qt-based browser (Arora) and file manager (Dolphin), editor (kwrite) and console (konsole) running, it uses 350 Mbytes. This is less than GNOME. I have no idea how Phoronix and Kubuntu manged to use up as much memory as they did.
BTW, KDE 4.4.1 running under Arch Linux, with composited desktop using the open source ATI 3d graphics drivers, is faster than GNOME on the same hardware.