Somehow I could no longer stand looking at the barebone case, so I bought a new LC-Power Pro-917B (http://www.testberichte.de/p/lc-power-tests/pro-917b-titus-x-testbericht.html). The case was not really expensive, but it still looks great for the price. Now I finally have more room again. Working inside the barebone had become rather fiddly over the years. And the computer was not doing much traveling anyway. Another problem was the temperature in summer, since the computer was running in my room under the roof (without air conditioning so far).
Here is a picture of the new case.
And so far everything is nice and quiet, too.