Applying displacement map to single face? (Noob issues with texturing)

How is this done properly? I tried playing around with the displacement modifier to see what these maps can do. Also added a subsurf modifier to get a lot of subdivision, so the displacement had the mesh to work with. Looked fantabulous. Problem: these modifiers seem to always work object wide, which seems to be of very little use when I try to finish my spaceship.

So I tried to learn a bit about cycles and found that you can use the displace input in the final node of a material. Tried that and it gave me a displaced texture. Problem here is: nothing happened in the render… the material preview looks fairly nice, but if I assign said material to a face, not much happens, even if I subdivide it manually just using the subdivision button.

Also: If I want it on onl yone face, can I still somehow get proper control over the displacement map and which parts are used? And is there an easy way to always get the original size of a face, and not the distorted versions that happen when unwrapping? Seems hard to get a proper high detail texture (fonts, painted windows etc … things, that require specific sharpness), if everything gets stretched again after?

Use the vertex group option in the displace modifier to enable it to be applied to just to specific faces assigned in that vertex group