I'm seeing the similar issues as fablom.
I, too, have a 6600GT card. I have a AMD Athlon 64 X2 4400+ and I am running a 32bit (K7) SMP kernel.
I had no issues when using Breezy (5.10)
I recently upgraded my Ubuntu installation to Dapper (6.06 I believe) in order to check out the compiz extensions (wonderful wobbly windows, etc.). That is when the issues arose.
1. Quake 4 would fail to load (seg. fault) just after a listing of the GL capabilities,
X..GL_ARB_texture_compression not found
signal caught: Segmentation fault
Trying to exit gracefully..
2. I noticed that I could open the nvidia-settings dialog. The attached screenshot is what I see. There is also (for completeness) text output on the console.
ERROR: NV-CONTROL extension not found on this Display.
ERROR: Unable to determine number of NVIDIA GPUs on ':0.0'.
ERROR: Unable to determine number of NVIDIA Frame Lock Devices on ':0.0'.
In my case, I have no errors in the xorg log file. However, the log file being written to is called Xorg.93.log which seems a little odd as I thought that the number on the log file would refer to the display (When I open a shell window, DISPLAY is ':0.0').
Perhaps it is a conflict between what is needed for compiz to work and the other GL stuff?
I'm going try reenabling the nvidia-settings package (which will remove the nvidia-glx package) to see what effect it has.