Somehow this thread did not pop up using quick links. Cypher's advise on the atmosphere settings are good.
One thing which used to be true, and may still be... When you work on areas of he planet away from the 0,0,0 reference, object placement besomes very difficult. I try to keep my renders in or near the default height field because of this. The object placement becomes difficult because of the way the object center is calculated in the populator (for populations). When you get near the equator, the object may calculate perpendicular to the surface. I may be wrong on this.