George and Squeak, you helped me a lot - thanks!

Squeak, choosing my post to answer was a very wise decision and is a great honor!
How are you today, hope the headache has gone?

Great to hear that both of you estimate the sound quality of the Casio equal or better than the PSR-550. - In the meantime I found on the net other demos and I read a lot about keyboards in my price range. I think I will go for the Casio. My personal reasons are:

I love the acoustic piano, saxes, organs, synth sounds(!) and I'm blown away by the E-Piano! Many other instruments seem to be good as well.
Many variations of the sounds (many more than the PSR offers), thats very important to me.
The DSP sounds really impressing!
Possibilty to download styles, voices and waves(!) through the internet.

Thanks again and take care,
Thomas