Boot into safe mode, hold command-s on startup.
mount -fy / (don't remember if it is mount_hfs or mount?)
chown root:wheel -R /System/Library/Extensions/Geforce*
chown root:wheel -R /System/Library/Extensions/NV*
You have to make sure the kexts are associated to root:wheel.
IF you want to go back to the 10.6.4 drivers; you can just copy them back in place from the location you backed OR use your time machine drive. If your using the time machine drive then you need to boot off an install disk and use the terminal that comes with the installer to copy over the 10.6.4 kexts over to your volume.
Now if you forgot to backup your 10.6.4 kexts and just went gung-ho, here are the 10.6.4 kexts. Use wget from the install disk terminal to grab them.
wget
http://victori.uploadbooth.com/osx86/nvidia-10.6.4.zip
unzip them and do the same as above, make sure to set root:wheel on all the kexts you move in place.
You don't need to re-install anything; just use a bit of technical savvy.
-- btw this *does* work, I been on 10.6.4 with the 10.6.3 kexts with no hard crashes all night.