I noticed that the Sushi (gnome-sushi) previewer uses around 60MB of RAM and wondered why it has to be so much memory, as I can only preview a single image/sound file at a time?
eog (eye-of-gnome) the default picture viewer uses around 30MB and deadbeef a lightweight music player around 10MB RAM and they aren't that much slower in showing up.
It's not a real solution to my question but it nicely solves what I wanted to accomplish.
I found Gloobus to replace Gnome-Sushi with this very nice intructions, which is lot more lightweight.