It already happened to me twice. I upgraded Fedora - once it was from F18 -> 19 on one machine and second time on F19 -> 20 on different machine. The symptomp was weird. I booted to GUI, log into KDM and enter in KDE session. Once session has been loaded (application restored, all autoload app loaded, panels restored) - bang - X server crashed.
I hunted and tested zilion of cases. But to make the story short: the cause is upgrade from xorg-x11-drv-nvidia-304xx to xorg-x11-drv-nvidia and in config /etc/X11/xorg.conf.d/00-nvidia.conf was left old path:
Section "Files"
ModulePath "/usr/lib64/nvidia-304xx/xorg"
ModulePath "/usr/lib64/xorg/modules"
EndSection
Just edit that path to:
Section "Files"
ModulePath "/usr/lib64/nvidia/xorg"
ModulePath "/usr/lib64/xorg/modules"
EndSection
restart X server (or reboot). And problem is solved.