Eduard - I just ordered a Coklin "P" size graduated filter set. They also have a very large verity of colors and types of filters so the possibilities are endless.
John - The rule of three was taught to me by a professional scenic photographer at a four day workshop in the NH White Mountains. It makes since to apply it to photography as well.
I should have posted the EXIF data with the photo: D90, 18-105mm VRII Nikkor, ISO: 400, CPL Filter, f/16, Shutter Speed: 1/20, HDR Photomatix, PP in Aperture 3.