Hi, Greg: It may be helpful to say what program you are currently using. I assume Fractal Terrains for one, but do you have access to any others?

Bryce is a decent rendering landscape program from what I can tell, but I've also seen tutorials online about doing some 3D rendering in Photoshop (and you can import DEMs via a secondary reader, I believe; once saw a tutorial about that somewhere...)

Someone recommended World Machine to me too, which may be what you're looking for: http://www.world-machine.com/download.html

Many great links for exactly what you're looking for are here, too: http://www.shadedrelief.com/

I hope this helps a bit!