The render I posted July 21th had for Atmo 40, Clouds 300. Not to bad I think. Maybe it is the fairly small size I rendered it, that causes the grainyness (?). The image was 1280x960, and to upload it I made it 760x570. I upload a new one now; rendered at 2560x1920 pixels and reduced it again to 760x570 pixels. It took a real long time to render, so I changed Atmo to 32, clouds to 128. Render time:
31 hours. There is some grainyness here and there, so higher quality settings in Atmo and Clouds still may be a good choice, but the improvement is considerable I believe.