Thanks for your feedback.
If the major problem is the X server configuration, can I find somewhere a "eay to use" description how I can handle it? To be honest, I haven't changed a xorg.conf file manually up to know.
PS: I have already executed the nvidia-xconfig, but the result was that the X server couldn't be started anymore. After that I executed sax2 which (well, this is at least my understanding) reinstalled the "Vesa driver".