Comment 8 for bug 970795

On Ubuntu 12.10 "Quantal Quetzal" and later you will no longer be able to use the proprietary fglrx driver on your Radeon HD 4850/4870 as AMD have dropped support for this card in their drivers around the same time that they added support to their drivers for the newer kernel and xserver used in Quantal.

You will be able to use the open-source radeon driver, but note that HDMI audio on AMD/ATI GPUs has been disabled by the upstream kernel developers as it causes blank screens on a lot of systems.

You can re-enable HDMI audio by adding radeon.audio=1 to your GRUB kernel boot options as described at https://help.ubuntu.com/community/Grub2/Troubleshooting#Editing_the_GRUB_2_Menu_During_Boot