My 2008 Radeon HD 2600 (RV 630) stopped working after upgrading to Ubuntu 12.10 “Quantal Quetzal”. X starts up nicely, the login screen is okay, but when I open a session there is no Launcher, no menu bar. That's because the newer AMD drivers don't support the older cards. So X reverts to plain VESA drivers, while Unity needs accelerated graphics.

The solution basically consists in downgrading X and installing “legacy” drivers (newer X.org releases are not supported by “legacy drivers”...). An easy solution is described here. It uses a Personal Package Archive (PPA) set up by Tomasz Makarewicz (praise him!):

sudo add-apt-repository ppa:makson96/fglrx
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install fglrx-legacy

To uninstall:

sudo ppa-purge ppa:makson96/fglrx

Update, 11 January 2014: since then, I have been able to switch to the Open Source radeon driver, which proved to work like a charm under Ubuntu 12.10, 13.04 and 13.10. More information.