Increase the number of X and Y triangles to get a more detailed map. The 'number of brushes' is the number of displacements that dispgen creates to model the heightmap.
So I made a heightmap in Terragen: And then used dispgen to create displacements that I tweaked and eventually looked like this: Check emp_dune thread for more pics and map download. I have to say this is a great way to make empires maps quickly that look good.
Ive written a tutorial on how to make displacement environments with terragen and dispgen: http://forums.empiresmod.com/showthread.php?t=6332