I had a similar problem w/ these drivers. Having a look at your xorg.conf would help. The thing that fixed it for me was making sure you have a DefaultDepth set in the Screen section.
CODE
Section "Screen"
Identifier "Screen 1"
Device "nvidia6600gt"
Monitor "sonylcd"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1280x1024"
ViewPort 0 0
EndSubsection
EndSection
Here is my Device section for refrence:
CODE
Section "Device"
Option "NoLogo" "true" # [<bool>]
Option "HWcursor" "true" # [<bool>]
Option "RenderAccel" "true" # [<bool>]
Identifier "nvidia6600gt"
Driver "nvidia"
EndSection
Under modules make sure you comment out dri and enable glx:
CODE
Section "Module"
Load "dbe" # Double buffer extension
SubSection "extmod"
Option "omit xfree86-dga" # don't initialise the DGA extension
EndSubSection
Load "type1"
Load "record"
Load "xtrap"
Load "freetype"
Load "glx"
# Load "dri"
EndSection
My full xorg.conf can be found
here.
I am using a 6600GT PCI-E card. You may need to set the NvAGP option correctly in Devices.