This morning I checked yum for system updates and the only two packages available were:

xorg-x11-drv-nvidia-1.0.9631-1.lvn6
and
kmod-nvidia-1.0.9631-1.2.6.18_1.2849.fc6

So, I ran an update. On the next restart of X server, starting server failed. I was prompted for a root password (which I entered) to let the system try to repair the configuration. The repair 'succeeded', but, graphics sucked.
The resolution I was using so far was 1152x768 with 75Hz refresh rate. Now, when I try to set the resolution on 1152x768 in KDE Control center, it works, but the refresh rate just won't go above 55Hz (graphics suck again). There is a very unusual line in the /var/log/Xorg.0.log for functional NVidia configuration:

(EE) Failed to initialize GLX extension (Compatible NVIDIA X driver not found)


Now, this is the second time I'm experiencing "XServer-Nvidia" problems after updating them from the official repositories - since installing FC6. The first time, restart solved the problem, since I wasn't very keen on handling this kind of problems at the time (nor am I now).
I'd like to solve them myself, but I just have more important things to do, and I'm afraid this is going to turn into phobia from "NVidia-Xserver" update.

Seems to me like the resolution/refresh rate won't work because the NVidia driver just isn't functioning. Right now, I'm running 1024x768 resolution with 85Hz refresh rate, and it's a bit hard to get used to it since the previous res. was higher.

So, what do I do to get this working again? Do I download driver from Nvidia site and install it, while removing present drivers from the system, or can it be done in a more elegant manner?

Ok, I've managed to fix the graphics. The resolution I previously used was more likely 1152x864, 'cause the 75Hz refresh rate can be applied with this resolution and not with the other one.

I've fixed this by replacing the /etc/X11/xorg.conf with another xorg.conf file which I backed up earlier this year.

Now, there are some other things puzzling me. For instance: when I set the refresh rate to 75Hz using "Nvidia Display Settings" tool (default setting is "Auto"), KDE Control Center reports the refresh rate of 54Hz?! Screen image looks fine to me... Doesn't look like refresh rate is too low. Is it possible that Control Center is not reporting it right?

The second thing which baffled me was the fact that when I changed the "Video Card" setting in the "Display Settings" (current value is Unknown video card) to NVidia GeForce... graphics got messed up again! Afterwards, there was no OpenGL rendering support.

Other things I'm still wondering: has anyone experienced such problems? Especially the one with updating.