Installing nVidia GT640 Graphics Driver (Mint 13 and Ubuntu 12.04)

Add the following lines to your repositories list...

deb http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu precise main
deb-src http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu precise main

By doing...

gksudo gedit /etc/apt/sources.list

And then import the GPG key with...

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 0x643dc6bd56580ceb1ab4a9f63b22ab97af1cdfa9

Refresh packages list in Synaptic and install.

More here.