Compacts are good. If you don't have a Petsmart nearby, you can get the compact bulbs and fixtures at a Home Depot or Lowes.
Alternately, you could buy two 2' long fluorescent fixtures (each holding 2 20W bulbs). Your tank is 20" long, so there would be a little overhang, but all of the light would be going into the tank since the fixtures are a bit longer than the bulbs. You can also buy these at HD/Lowes.
All said, you should have at least 60-80W of fluorescent lighting over the tank.