It also depends on how much time, effort and money you are willing to put into your tank. If you go the high light route at 3 watts/gallon or more you will need to consider the cost of a pressurized CO2 system. As Fishsticks said, the types of plants you want will determine the amount of light you need. I've got a 29 gallon tank with a 65W CF fixture and it gives me all of the growth that I can handle. Most of my other tanks have even less light.