Thank you very much, Zanders: this appeared to do the trick:
Option "DynamicTwinView" "0"
Now I have one more question: my screen is still blurry; it is not so crystal clear as it is under XP (look at the fonts for example). Would you happen to know how I can solve this? Currently it's running at 1280x1024 @ 75Hz. Since the three screenprints are to large to attach to this message, I've uploaded them to mytempdir:
Thanks again for your help ;-)