Another article, more descriptive.
EDIT: You know what ? Fuck the above 2 guides for wasting my time. They’re missing steps here and there.
If the GPU used for display is an NVIDIA GPU, the X server configuration file, /etc/X11/xorg.conf, may need to be modified. In some cases, nvidia-xconfig can be used to automatically generate a xorg.conf file that works for the system. For non-standard systems, such as those with more than one GPU, it is recommended to manually edit the xorg.conf file. Consult the xorg.conf documentation for more information.
So there you have it – after installing the CUDA 7, run:
/usr/local/cuda-7.0/bin/nvidia-xconfig to generate a new xorg.conf file for X Server. Else your X Windows configuration may not know about the new CUDA 7 driver.