if you put a higher watt bulb in the fixture it may burn the ballast up or it wont last long, youll have to get a different fixture to get more lighting
You could get an AHsupply retrofit kit for your hood.
Get the 36watt kit shown here: http://www.ahsupply.com/36-55w.htm
Or if you are interested, I can sell you a custom 65watt 6500K CFL light I am currently using on my 10gal (I'm upgrading tanks, so I need to upgrade the lighting too)