I was having similar problems with this release on a similar video chip.
Try booting into CLI mode and log in with your use name and password.
startxthere at the termina.
If the GUI loads, then you have the same problem I did, which is a broken KDM package.
If that works, simply su to root, then slapt-get -i kdm-small
Reboot, and you should have your GUI back.
PS. IF startx doesn't load the GUI, then you have a different problem.