I've just done some experiments to create DTMs.

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Inside the red box there is a resolution of 20 m per pixel, with shadows, outside 80 m per pixel (very rough...).
I used GRASS GIS to get the DTMs, unfortunally I was not able to get them by Quantum GIS.

My final map will have a scale of 1:200000, so I think that the 20 m resolution will work well.

Please, let me know what do you think about.