Just to add to Oshyan's last post, for people who may be reading this and are new to these concepts. The issue Oshyan explains is NOT a fault that terragen has but other soft is without. In maya and mudbox for example, you do the same as Oshyan directed. That is, The higher the geometry level of the base model the better the model will take and represent the maps. only the balance a user needs may be different.
But where Maya (again just for example) may have more tools to make things of this nature work, Terragen does not need them. IN Terragen you can populate an entire scene with the high polly and no need for simulated displacement... Literally globally (if you have the system to handle it) something Maya CANNOT do.
But really, once you get beyond the foreground, its not necessary to have real displacement or geometry, then bump will usually do fine anyway. My point is that this give and take that Oshyan describes seems universal across all 3D software I have seen so far. And again, Im just saying so, for any new users who come across this thread.
As someone still relatively new to 3D my self, this was an important lesson for me to learn. And once I finally got it through my thick scull, I was able to start making the images I wanted to.
Just saying.
