dpkg-reconfigure xserver-xfree86
Unless it is some other package in Mephis. Answer the questions.
Restart X and see what happens.
And if you are willing to do a re-install again... DO try the Debian-Installer. It is fantastically better than anything that was there before. They have a TEAM working on it. Main reason -- complaints Debian wasn't very easy to install.
It is still textual install, mainly because it runs on 11 different platforms (actually 15 but that is another story).
It has got ump-teen translations, works very well indeed for most run of the Mill machines most Linux machines turn out being... Even does work on Newer motherboards and Video cards. Plus it has the 2.6 Kernel as an option rather than an experimental flavor. 2.4 is default.
You never know. Sarge *IS* shaping up very well, the longer they work on it.