Clouds (both 2D and 3D)

As I said before 2D clouds render extremely fast but still look great unless you're doing a flythrough. Just to make your life a little easier, I'll post some pictures of 2D and 3D clouds and the render times so you can actually see the difference. My machine is a P4 2.5GHz, and 1.3GB RAM.

2D clouds

1min 15sec                                    17sec                                                1min 20sec

(Real Earth+Reddening)           (Flat Earth+Reddening)              (Real Earth+Reddening)

3D clouds

As you can see, 3D clouds look much nicer. They are much denser and have more depth than the 2D clouds. With these you can take your camera anywhere and fly through them. I'd recommend using 3D clouds only for low clouds. You can get away with using 2D if they're high up.
In the CG world you always sacrifice speed for quality. The better it looks, more likely the longer render times. I'd also like to add that the 3D clouds in DS take longer to render than if you have done clouds using AfterBurn.

8min 56sec                                     8min 7sec                                          7min 15sec 

(Real Earth+Reddening)            (Real Earth+Reddening)              (Real Earth+Reddening)