Vector will run very nicely on that machine. It uses light window managers and is only a 235 mb download. www.vectorlinux.com
Just noticed the KDE/Gnome point - you can install KDE or Gnome after you download the main distro, or try their 3.2 SOHO which is also excellent and comes with KDE as standard.
By the way, why couldn't she use ICEWM, for instance? It can look very like KDE and it's easy to use.