Hmm, I think I would look around this site and when you see images you like, look at the artist information to see what software they are using. That might help?
It also depends a lot on what you want in your final image. If you like to basically "stamp" down beds, lamps and so on or if you prefer to draw them all yourself. That can have a lot to do with what you need software wise.
Since you mention dungeonographer you might check out their gallery and see what others have done.