Comment 63 for bug 565883

Yes, this is understood. However the artifacts appear when the radeon driver installed. Even with having the Acceleration disabled (System > Preferences > Appearance > Visual Effects > None) the screen has some artifacts (see screenshot visual-effects-disabled.png). However they don't cover the whole screen.
I suppose, I need to buy another graphic card for this computer in order to use it properly with Ubuntu.
Thanks again for your attempts fixing the problem.