Not living anywhere near Houston I can't be of much help myself, but I'll do what I can starting with publishing this question. There, done.
You might be able to connect with "prominent" atheists in your area by contacting local atheist groups. Failing that, those groups would all be glad of some publicity and could each supply very knowledgeable spokespeople. Check out the Houston Freethought Alliance, whose front page has links to all other such Houston groups I was able to find.
If I were in your position, I'd do a piece on morality centred around the phrase, "You can be good without God." A lot of evangelists and other apologists argue that religion alone is holding society up with its moral guidelines, and that everything would collapse without it. This actually has no bearing on whether a god exists, but they use it anyway as an appeal to consequences. By outlining sensible secular systems of morals and ethics, you can help to dispel this idea.
YouTube has a fair few recordings of Richard Dawkins holding forth on many American radio stations. Though many of the callers are attempting to attack evolution, given that it's him, these recordings will still give you some idea of what you'll get if you take callers.
Good luck. Let us know what you do and how it goes.