Identifying Your Graphics Chip
First, check your graphic card name and chipset:
sudo update-pciids #optional command, requires internet lspci -nn | grep VGA
It should report something like this for your graphics card:
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV710 [Radeon HD 4550]
Testing The Driver
To look for boot messages/errors, check
dmesg | egrep 'drm|radeon'
To see your OpenGL information, you can run the commands below. Make sure your OpenGL renderer string does not say "software rasterizer" or "llvmpipe" because that would mean you have no 3D hardware acceleration:
sudo apt-get install mesa-utils LIBGL_DEBUG=verbose glxinfo
Typically, the following manual commands will properly uninstall -fglrx:
sudo apt-get remove –purge xorg-driver-fglrx fglrx*
Fonte: https://wiki.ubuntu.com/X/Troubleshooting/VideoDriverDetection#Problem:_Need_to_purge_-fglrx
Recommended configuration for X.org
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core sudo dpkg-reconfigure xserver-xorg
Boot into recovery mode and select Root Shell. Then run:
X -configure
Then:
cp /root/xorg.conf.new /etc/X11/xorg.conf
Reboot and you can edit the new Xorg.conf.
Fonte: https://help.ubuntu.com/community/RadeonDriver