I am setting up my first drupal (6) site and so far I like the system.
I've now run into a problem however: to give the content more space I want to hide the right-sidebar (with the navigation menu etc.) on every page from a specific module (or also fine: for a specific node/view from the module)
The only way I've come up with is to add some CSS to the module CSS files, but this doesn't seem very clean to me since I would need to redo it on every update (also the module uses 5 CSS files for different views)
Is there a better way to do it?
To be clear: I don't want to just hide a block, I want to hide the whole sidebar
Hmmm. Is this a custom module or something commonly available? Some modules allow you to create custom tpl.php files (see the theming guide) for them. This might quickly solve your problem.
If you are theming a node, then it is significantly easier. You can theme a specific type of node by naming convention (again via .tpl.php files in your theme). You can check out how to do that here.
Definitely check out the theming guide, since you will probably want to create your own theme instead of hacking on one of the core themes (not recommended). Typically you can copy /themes/garland into /sites/all/themes/my_garland, switch your site's theme to that and then make whatever changes you need (otherwise you'll have to reapply changes every time you update core).
Finally, you can check for path arguments (which seemed weird to me at first) if you want to do things in PHP that are more complex (see the arg function). If this is your first Drupal site, you may also need to know how to include css programatically.
Good luck! Drupal is a fun and interesting product.