I have similar config, but I don't use the XfbDrake.
The config file you have to edit is /etc/X11/XF86Config-4
You can find help on it by typing 'man XF86Config-4' in the console.
Also have a read of README file from nvdriver ( http://download.nvidia.com/XFree86_4...960/README.txt
If you have troubles to do with this, ask me (icq 161866753).