Since you can log into root,my advice is login as root and try to revert to previous state. If this doesn't work, apt-get -t testing install kdm or run synaptic or kpackage and look for newer version of kdm.

If that doesn't work and you would like to try GDM, then
apt-get install gdm.
choose to use gdm as default windows manager.
or run dpkg-reconfigure gdm
There will be gdm utilities menus created, so you can set your gdm the way you like.
You might want to remove kdm afterwards: update-rc.d -f kdm remove