when you do /sbin/modprobe NVdriver, this try to load NVdriver. If it is load, does nothing.
If you put modprobe , alone, it does nothing.
try /sbin/depmod -a ; to check dependencies between modules.
when you install Nvidia Drivers, rpm, srpm,tgz.. XF86Config doesn't change.
You need to change Driver "vesa" or Driver "nv" to Driver "nvidia" manually.
Also need to add Load "glx" and delete (Load "GLcore" and Load "dri")
Look at my Config, as an Example. Search IMPORTANT and you will see my Changes
Also need to verify that /etc/modules.conf or /etc/conf.modules contains a line "alias .... NVdriver", But rpm ever make this change