Heh... It really depends on what you want to grow in your tank. In my west african tank (a 29 Gallon, 75cm x 30cm x 40cm approximately) I have a single 20 watt bulb (hagen PowerGlo).
Now I am only growing Anubias Nana and a few other Anubias species, but you get the idea.
If I wanted to grow anything else... Like Glossostigma, or Dwarf Swords.. or any other carpet plant, I would have to get much stronger lighting... Probably a pair of 55 watt CF bulbs...