hey these are great, Leisha!
I've not actually used Terragen yet - got Mojo World creator and haven't used it either - not enough time! - but I like your results. Bryce 5.5 is also available for free.
The Cool thing about using these kind of programs is that it help immerse yourself into the world you create. You can do just as Leisha has - create a 2d map and also the 3d world. Then you could stick your 3d renders to areas you this suits parts of your2d map - as well as doing this with photos you can find on the web.
I'm taking all this a step further with 3d packages - like Maya and 3DS Max, or a Free Noncommercial product
- one of which is the Maya P.L.E (Maya is the premier 3D software used by design industry , film companies. Game Companies use either Maya or Max )
Bryce is another tool for environments and there's a free version in the above link.
(BTW that above site was the first I came across while typing in 'free 3D software' in Google - it's got a some great SFF illustrations on it for your desktop. However some of the links from it seemed suspect. something tried to down load itself. dunno what it was and stopped it, but I don't trust it 100% )
with a 3D package imagine now being able to build your ideas for castles and towns in 3d - make them come to life. I'm doing this with one of my locations the more I work on it, the more I think abut possibilities with in - where would the king live? where would the guard room be? the people of importance down to the common folk? would their location have a bearing of them appearing in a scene in the story? what is the architectural influences and how does that reflect on the culture of your people? You can get a camera in there and be in amongst it.- it all helps
Before long, you'll have built up loads of references to know your world even more. Describing it then because that more easier for writing because it's there in front of you.
...also when Hollywood comes knocking (well Peter Jackson ) you can show them all this stuff to their art department!