Slow down and breath...
The opensource driver s of XServer rarely need to be "re-installed." Problem with those instructions you used, is that there is 3 different opensource (xorg) drivers specifically for ATI cards and you need to match them to the Vidoeo GPU you have...
From your 2 posts, I assume (please confirm or answer) that you had a working install of Xubuntu on a computer that had Radeon graphics, right? And you had one of the Radeon drivers installed? All worked fine until a kernel upgrade?
* If the boink occurred with the kernel going from one version to another, you need to reinstall the driver on "that new kernel version"...
Problem now is, that you've already made changes, so we need to back then off.
Lets start fresh as is that didn't happen.
Boot and use the rescue menu item for the current (new) kernel version. Use root menu item. At the root prompt, you are going to mount the filesystem in read/write. The rename the xorg.conf file to "some other" name.
Okay, so the xorg.conf tells Xserver which video driver and options to use. Renaming it to something else, will put it in a mode just like it was a fresh install // to find on it's own what is there. As a Radeon user, you know this doesn't work well without helping it along by giving it hints.
mv /etc/X11/xorg.conf /etc/X11/xorg.conf.old
Read this before leaving root prompt:
You could reboot right now using a "radeon.modeset=0" boot parameter from the grub menu, then install from one of the additional drivers utilities, which will work fine for about 90% of users... but if you are one of those last 10%...Here's the problem there- The additional drivers utility looks for what video driver is installed by assuming that it is loaded. You changed the xorg.conf file, so it may or may not know one it really installed.
It may say one of two things- If it says that it is installed is installed and not active... remove it then install it again. Should work fine (95% of the time). If it says it's not installed... it got fooled. The problem with past experience with this, is sometime installing a new driver "over" and old driver without removing that old driver, is that it may have mixed version fragments or dependancies versions that go along with them that don't like to play well together. Better to remove the old driver, before installing the new. The problem with a kernel upgrade, it that if you just try to install over the same version, it looks at the package version and says, it's already out-to-date.... So we remove... then install.
So while still at the root promt... we can remove the radeon drivers while we are still there, so we can start fresh , to install.
You could now go gui to install your driver (if you used a "radeon.modeset=0" boot parameter from grub... But since you are in a root prompt, it's only a few more keystrokes away to do it manually... This assumes the fglrx packaged driver supports your radeon card version...
# This is a comment You don't have to in these comments...
# Freshen/update the apt cache
sudo apt-get update
# If you installed a ATI binary driver at one time...
sudo sh /usr/share/ati/fglrx-uninstall.sh
sudo rm /usr/share/ati/fglrx-uninstall.sh
# If you have an ATI packaged driver installed
sudo apt-get remove --purge fglrx*
# This is new
# now copy down what the kernel version says and substitute
# that text for the string $KernelVersion in this next command
sudo apt-get install linux-headers-'$KernelVersion'
sudo apt-get install linux-headers-generic
And you should be going again.
# Now all is clean and set up for the driver install
sudo apt-get install fglrx
sudo apt-get upgrade
sudo shutdown -r now