More possibilities to try

Disable xorg.conf (Squeeze doesn't normally need it)
mv /etc/X11/xorg.conf /etc/X11/xorg.conf.renamed

apt-get update
apt-get -f install

dpkg-reconfigure xserver-xorg

If you use a proprietary graphics driver, have you reinstalled it?

"KDE-Trinity 3.5.13 hung my machine" might not be quite correct, maybe 
more to do with a xorg/graphics related issue in the Debian upgrade