Now, i searched and found a solution for me, don't know if it's the correct way but it runs.
first i add in the Section "Files" the following two lines, so that the X-Server picks up
the correct libglx.so. Without this change the one from
/usr/lib64/xorg/modules/extensions/libglx.so (xorg-x11-server-Xorg-1.10.4-1.fc15.x86_64) was taken.
and at last i moved th libglx.so from the X-Server away, so that it could not be taken.
mv /usr/lib64/xorg/modules/extensions/libglx.so /usr/lib64/xorg/modules/extensions/libglx.so.xorg
the Xorg.0.log files tells me now that the correct lib form /usr/lib64/xorg/modules/extensions/nvidia is taken.