I have made a big step ahead. The key was "hardware acceleration" of my GF 6600 graphics card.
Hardware acceleration was on low, because my projector wouldn't display XviD and DivX-Movies. The projector runs as clone on the second graphics port.
After putting hardware acceleration to maximum, several ratdvds are running now! (Although it is not really stable and single ratdvds still don't run at all).
The videos won't be displayed on the cloned image on the projector. Dual screen mode though does the trick, i.e. ratdvds (and Xvids, too) are displayed correctly.
I hope I can help some of you with this information.