The ATI driver available at Ubuntu Additional Drivers don't install correctly. To fix this issue, install the latest stable version from the ATI suporte site and you'll get the AMD Catalyst Control Center, a GUI with an option to Switchable Graphics.

To install it, follow the instructions below.

1) First, check the software sources, then paste this command into the terminal to install the dependencies:

sudo apt-get install build-essential cdbs dh-make dkms execstack dh-modaliases fakeroot

1-A) ONLY IF you have a 64 bit system, do this EXTRA step:

sudo apt-get install ia32-libs-multiarch:i386 lib32gcc1 libc6-i386 ; cd /usr ; sudo ln -svT lib /usr/lib64

2) Paste these commands into the terminal to create a new folder and download the latest catalyst driver:

 mkdir ~/amd-catalyst-driver; cd ~/amd-catalyst-driver/

 wget -c wget -c http://www2.ati.com/drivers/linux/amd-driver-installer-catalyst-12.10-x86.x86_64.zip

 unzip amd-driver-installer-catalyst-12.10-x86.x86_64.zip

3-A) IF you have Intel HD graphics AND discrete ATI GPU, then do this, click NEXT > NEXT > etc (and reboot):

 sudo sh amd-driver-installer-catalyst-12.10-x86.x86_64.run

3-B) ONLY IF you have just the ATI Graphics card, then try to do this instead, but it could take a while:

 sudo sh amd-driver-installer-catalyst-12.10-x86.x86_64.run --buildpkg Ubuntu/precise

 sudo dpkg -i fglrx*.deb

4-A) Overwrite the /etc/X11/xorg.conf file(anyway):

 sudo amdconfig --initial -f

4-B) Force the use of the new xorg.conf file:

 sudo amdconfig --input=/etc/X11/xorg.conf --tls=1

6) Reboot the computer and you're done:

 sudo reboot

7) To verify your installation, in terminal type:

 fglrxinfo ; fgl_glxgears

8) To switch between the GPUs, run one of the following commands and re-start X to take effect

8-A) Activate discrete GPU (High-Performance mode), must re-start X to take effect

 sudo aticonfig --px-dgpu

 fglrxinfo ; fgl_glxgears

8-B) Activate integrated GPU (Power-Saving mode), must re-start X to take effect

 sudo aticonfig --px-igpu

 fglrxinfo ; fgl_glxgears

SOURCES:

http://blog.wapnet.nl/2012/07/ubuntu-12-04-install-ati-videodrivers/
http://wiki.cchtml.com/index.php/Hardware
http://support.amd.com/br/gpudownload/linux/Pages/radeon_linux.aspx
http://forums.linuxmint.com/viewtopic.php?f=191&t=112234
http://ubuntuforums.org/showthread.php?t=1930450