Thank you for all the nice comments and suggestions.
Regarding the clouds I might add, that they look in fact bland in the original image, and are nothing exciting (as far as the setup in Terragen goes).
I am an avid fan of HDR photography and my interest in Terragen mainly came from not being able to travel much at the moment and rather use Terragen to create my HDR sources. After the first test with the .raw image that Terragen puts into the temp folders didn't work too well, I set up an animation of 5 frames, and keyed the exposure of the camera to 0.0625, 0.25, 1, 4 and 16 (multiples of 4, to get something similar to an EV-spacing of 4). I then used photomatix pro to combine all the 5 renders into a HDR image, and then remapped it to the desired effect.
As you can see, the sky is not too exciting, but the HDR remap works amazingly well on pictures created with Terragen.