Though I almost exclusively use under belly heat in my snake room I do like radiant heat either from a bulb or a heat panel. In my case space considerations just make underbelly more practical.
If you want to use a bulb the first rule is that there are no rules. There are just too many variables like size and dimensions of the cage, ambient room temperature and ventilation.
The second rule is get yourself a thermometer and do some experimenting BEFORE putting the snake in the tank. After awhile you'll get a feel for what works for your conditions.
As a general rule if using a bulb for heat I go for the red bulbs. Some say snakes can't see the red light. I don't know if this is true or not but mine seemed more relaxed when not subjected to blaring white light.
I would also suggest starting with the smallest wattage spotlight you can find. Place it on one side of the cage and start measuring what kind of gradients you are creating. Remember that you will have both vertical and horizontal gradients.
Another thing that I liked to do was to put something with some thermal mass directly below the lamp. By doing this the object can absorb heat when the light comes on and radiate it back into the cage when it goes off. Haven't really gauged the impact of this on the animals but the idea is to somewhat buffer the sudden spikes and drops as the lamp cycles on and off.