There are no formal graphics card requirements for TG aside from OpenGL compatibility. But for some reason even the basic OGL stuff we do in TG seems to work and then not work, depending on the graphics driver version! I suspect it may be because the graphics card companies are constantly trying to optimize for the latest games, sometimes making changes in how some things may be handled, and they don't test with Terragen or most other software, just with the games they're targeting and a few other specific applications most likely. So they never see these crashes, even though it's clearly something they're doing that's causing them (which we know because some versions of a graphics driver work fine on the same card where other versions do not).
In any case check to make sure you have the latest graphics drivers first.
To disable the 3D preview you can just close it with the X (close) button.
- Oshyan