You can use graphics card AA if you completely disable Post Lighting (not just AH3 AA). Then you have to set the graphics card AA to "Override Application".
This is a completely wild guess, but I suspect AH3 AA works at screen resolution*, i.e. it smooths the screen resolution which results in something less than your screen resolution. Graphics card AA does multi-sampling: it calculates the image at each point at higher than screen resolution, then averages those down to screen resolution. In essence for each pixel on your screen the graphics card calculates multiple sub pixels at higher resolution and then averages them to get the screen pixel. This is an over simplification; some settings just do that on polygon edges, some settings, which are much slower, do it for the interior of polygons as well, i.e. the textures. The latter is often not necessary if the textures have a good set of Level of Detail (LOD) images, i.e. they are affectively already anti aliased.
*whatever the explanation, on my system the graphics card 4x adaptive MSAA gives much sharper images than the AH3 AA with Post Lighting on.
