eeeexcellent.
Thats easy peasy to fix. You wont need to reinstall. All you have to do is restore the config file. Because it created a backup for you, thats not going to be a problem.
You might wanna write this down or print it or something because your gonna need to remember it.
FIrst up, boot into ubuntu and wait until ti all goes wrong.
Then press Ctrl-Alt-F3 this *should* take you to a terminal prompt, you can login here. Your now using Ubuntu without X and Gnome.
At this point we want to go to the directory of the xorg.conf file. so we enter the command
cd /etc/X11
That X has to be capital, or it will compain that the directory doesnt exist.
Now, enter this command to list all the files in the directory...
ls
Form this point ya kinda have to figure a few thigns for ureself. You need to replace teh xorg.conf file with an older one. But, you wanna do this as safely as possible, you might need your xorg.conf later on. So, take a look at the list of files, there should be an xorg.conf backup there somewhere, its got a huge name with loads of numbers in. The numbers are the date and time at which it was created. IF ya can find one all ya have to do is remove the broken xorg.conf and replace it with a new one.
so, backup ure current xorg.conf...
sudo cp xorg.conf xorg.conf.broken this just creates a copy with the .broken attached.
remove the old xorg.conf using...
sudo rm xorg.conf
Then copy over ure backup
sudo cp [i]yourbackup[/i] xorg.conf
Then all ya have to do is restart your computer in a nice safe fashion...
sudo shutdown -r now
Hopefully, when it restarts, you should have your GUI back!!!
Good Luck, if ya get stuck, nogg3r5 at gmail dot com is my MSN and email address.