In this day in age, the very old WPG rule is no longer an effective measuring stick to judge light adequate quantity. With so many lighting choices and much more advanced reflector technology, every situation is different. Add CO2 to the mix and you can effectively & significantly increase the effectiveness of your existing lighting per say. A simple flow chart would look like this (from least to most):
T12 --> T8 --> T5 --> PC --> T5HO --> MH --> HQI
Reflector quality can make a huge difference. Quality reflectors on T12's will result in plenty of light to grow most plants in many situations. Halides are typically far too strong for a planted tank and are only used in short bursts. Personally, I don't understand why you would want to bother with them. They look cool...for the very short amount of time you can run them.