Now it works.
Removed the second screen, reset the xorg.conf and installed the nVidia driver and then it all worked.
Except that I can't change the resolution with xrandr -s anymoore. Why?
Realy anoying. Want to be able to do that cous I'm trying to fix up a mediacenter which can be controlled with my Wiimote. Then beeing able to change the resolution from the wiimote is important cous I have a 16:9 TV and a 4:3 screen at my desktop. And the only way I know how to do that is trugh xrandr.
micke@micke-desktop:~$ xrandr -s 1280x768
Failed to change the screen configuration!