When I tried to install the driver for the NV card, things started to go wrong, because the NV driver replace the mesa OpenGL libraries with theirs. Because the xserver actually works on the Intel video card, which is incompatible with the proprietary openGL libraries, openGL stopped working and as a result, also Unity got broken.
After a couple of evenings trying to make it work, I found that the trick to make all be working is, after the installation of the nvidia driver, reinstall the original mesa glx libraries. Here is the right procedure:
* Uninstall all NV components on the system
# sudo nvidia-uninstall
# sudo apt-get purge nvidia*
* Download the last official drivers from the NV web site: http://www.nvidia.com/object/linux-display-amd64-319.60-driver.html
* Install the required packages for the installer
# sudo apt-get install linux-headers-`uname -r`
* The next steps requires the xserver stopped. So, Go to a terminal (Ctrl + Alt + F1) and stop the xserver. For instance,
# sudo stop lightdm
* Install driver
# cd path/where/you/save/drivers
# sudo chmod +x NVIDIA-Linux-x86_64-319.60.run
# sudo ./NVIDIA-Linux-x86_64-319.60.run
* At this point, the driver is OK, but our xserver would show problems with the Open GL libraries. However, the GPU is ready:
# sudo nvidia-smi
should show the status of the GPU.
* Now, to avoid the problem with the Open GL libraries, we (re) install both our xserver and the mesa libraries:
# sudo apt-get install --reinstall xserver-xorg-core xserver-xorg-
* Reboot the system
Now, you can install the CUDA Toolkit, avoiding to reinstall the NV drivers...