I read somewhere that we would get better result with Adsense if we place Adsense ads on several "average" websites than if we place them on one "top" website. Is that true? What do you experience?

It depends on how much traffic you're getting and if the ads look appealing to the visitors. Of course, having ads on more sites will give you more coverage however having ads on one site might yield better results because there will be more traffic.

Try making three sites and getting them to the top of Google... make them look different and have different content (at least a variation). This way if nobody goes on the first site they might go on the second or third.

Thanks for the advice. Creating three website sounds like a good start. If the sites run well and got nice traffic, we can create more websites and duplicate the promoting techniques.