I'm thinking of using Terragen, an on-the-fly 3D enviorment rendering progream, for my "concept art" of enviorments. I figure its all orignal art work, and would really get the feel of what i want me game world to look like, rendered in actual 3D, besides something I'd sketch. My question is, is it a credible program to use for this, or does every thing its incredibly amatuer?

Sure it's 'credible'. Anyway, it's only concept art, so why worry? Concept art is simply to let people know what your ideas are, and as a source of inspiration for those who have to actually create the game world. It can be in a completely different graphical style to the finished product. Terragen is to my mind the most realistic terrain rendering program out there.