Ok, try this then:
- Remove the 8400 card from the system and restart.
- Ensure the xserver-xorg-video-nv package is installed. Uninstall all nvidia-glx packages.
- Uninstall the nvidia driver package you downloaded by doing the following (I know this will kill your 3D, but it's only temporary.) :
Step 1: Log out of X or boot the computer and don't log in to X, switch to a text terminal by using Ctrl-Alt-F1 and log in.
Step 2: Shut down X by typing the following -
sudo /etc/init.d/gdm stop
Step 3: Uninstall the nvidia driver
sudo /usr/bin/nvidia-installer --uninstall
Step 4: Restart X
sudo /etc/init.d/gdm start
At this point it may come up and complain about being in low-video mode etc. Go ahead and reconfigure graphics and then select default generic configuration. After you click ok on that and it cycles back to the other menu click cancel and X should restart. Hopefully you should get a normal desktop back at that point using the nv driver.
If you're able to get to the point where you have a good desktop using the onboard 8200 and the nv driver, go ahead and reinstall the 8400gs and reboot. You should just have to reconfigure once like before and it should work as well. Let me know if this works and then I'll post more instructions on how to check for old kernel modules and reinstall the 180.44 driver. I just don't want to bury you in if-then type stuff just yet.