My idea is to get some kind semi realistic lighting from a 2d top down point of view. This would be something I would like to eventually use in my Warspace game, so when a spaceship flys past a lights source like a sun, or some explosions, the ship will be lit up.

 This is the texture of the ship i'll use, It's a 3d model rendered from a top down view in Blender.

 And this is a render of the normals, Blue for one axis, red for another, and green for another.

There is a tutorial on how to do it http://feeblemind.tuxfamily.org/dotclear/index.php/2005/08/23/43-tutorial-blender-and-normal-maps

With XNA, I get the colour from this image and work out what angle it is relative to the light source, so if there is a light source on the left side of the ship, then only the parts of the left side facing the light source will light up. At least that's how I hope it will work.