I would really encourage you to begin thinking about upgrading your pc... The fact is, in its current version, TG2 renders slowly. If you want this scene to work, you should do a full render, with higher detail settings and as child@play said increase the atmo samples.
This will take several hours, but if you have the time, or if you are willing to make a sacrifice, you could end up with a great scene if this preview is any indication!
Good luck...