I did a quick test using the Gradient Fill tool in PhotoPaint. I had the Steps set to about 80. That looked pretty good. Didn't need any posterizing. I'm not sure how many courses of stones the actual pyramids have. Must be hundreds. Don't forget to set the terrains Grid pretty high (512, or 1024).
FYI, the sides of the pyramids slope at 51 degrees, which means the height is about 62% of the width. They were originally covered with a smooth veneer of white stone. The pyramid of Khafre still has some of that veneer on the top of it.

Rubycon