You should use a method to install the driver which was made to work specifically with Ubuntu. I see that you did try the default method though...
I've never had an ATI card so I've never used their drivers so I'm not sure, but maybe the drivers on the ATI website has an uninstall feature? If it does then you can try to install it manually.
Worst case scenario is that you'll have to create Xorg config files where you specify the driver manually.
Edit: maybe you should create such config files now already so you can easily change between drivers later if something goes wrong. I wont be much help with that though because they have changed the way Xorg is configured lately... It all used to be in /etc/Xorg/xorg.conf but now it's spread out into different files, like one for the monitor, one for input devices etc. It's much more confusing nowadays in my opinion even though the opposite was the goal of the change.
I've never had an ATI card so I've never used their drivers so I'm not sure, but maybe the drivers on the ATI website has an uninstall feature? If it does then you can try to install it manually.
Worst case scenario is that you'll have to create Xorg config files where you specify the driver manually.
Edit: maybe you should create such config files now already so you can easily change between drivers later if something goes wrong. I wont be much help with that though because they have changed the way Xorg is configured lately... It all used to be in /etc/Xorg/xorg.conf but now it's spread out into different files, like one for the monitor, one for input devices etc. It's much more confusing nowadays in my opinion even though the opposite was the goal of the change.