you could also use a radiant heat panel. in the long run they are far cheaper to operate.
Ive heard good things about
http://www.pro-products.com/. you have to order directly from them.
"In order to determine the correct panel we need to know the size of the cage, what it is made out of, how much ventilation and where it is installed, what type of animal is in the enclosure and what the ambient and low temperatures are in the room where the cage is kept."
If you send them the above info they will tell you exactly what you need.