hello micha,
you are right, you need the legacy package. Why didn't you install it?
Please don't expect that apt will decide which driver to use.
apt will never upgrade from the "nvidia-driver" package to the nvidia-legacy-340xx-driver package.
The install procedure is in the other nvidia thread, where you already posted.
see
http://forum.siduction.org/index.php?topic=5447.msg44469#msg44469greetings
musca