I can confirm kjaras solution! Now Steam is running also on my computer, thank you kjaras!
For Newbies like me, I want give more details:
If you have found the entry "nvidia-driver" with label "nvidia-megapackage" in that giant list of the synaptic package handler and you have started the process, the programm startet a 'big' download over 30mb and execute the installation; -Don't disturb it.
Then reboot the computer.
In my case there was an additional problem: The Steam-Client did not want to start as root (it identify me as root). Perhaps because of the automatically installation of Sparky, which allows me as user to do administrator things...
I could handle that with the command 'sudo -u goofy
steam' (in the LXTerminal). goofy is my username and must individually changed.
And then the Steam-Client is running. It begans with download of 270mb! for an update. After this update the Steam-Client is running by clicking the icon, without any problems.
What I have misunderstood apparently is, that the programm updates with APTus doesn't include the Steam-Client. And the pre-installed Steam-Client was one important reason for me to select the Sparky-Linux...
Now, it works and its great; But I think that Story is not the best advertisement for Sparky. The post from kjaras is from August 2016, and the problem is still the same...