I've noticed that you are using scummvm 1.6.0 and not 1.7.0, i have a 16:10 monitor. If i want to play in 4:3 at fullscreen, with black borders bars at the side of the window, natively with scummvm, and not forcing by nvidia panel, i must to edit atlantis.ini and add "gfx_mode=gl1" (or gl2), so i must to enable opengl. Ascpet ratio=true is already setted, so no problem at all. But in this way, yes, we obtain a 4:3 image , but without any kind of filter...

With scummvm 1.7.0 they add an opengl driver with filters: gfx_mode=opengl_linear

differences between scummvm 1.6.0 gl1 and 1.7.0 opengl_linear:

http://oi57.tinypic.com/2ic8d2e.jpg

http://oi59.tinypic.com/2jdmd7p.jpg

both images are at resolution of 1280x800 with the right proportions.
Basically you can accept even the normal and squared visual rendering, but with 1.7.0 you can obtain both the renderings and choose the one you like more (i'm talking of people with 16:9 or 16:10 monitor that want to play in 4:3).

Thank you! Good to know.

I prefer the sharper pixels :)