I have a 20 Gallon High as well and I didn't stick with the a florescent like you mentioned when it came to growing plants, so I can't say for certain whether yours is enough.
What I have had success with was to use a one of the slim factor NO T5 fixtures, over a glass top. Recently I added another one. Before and now I haven't had any light trouble with: Crypts, Baby Angel Hair as a carpet (probably the item requiring the highest light for my tank), Java Fern, and Ludwigia. None of these are really high light requirements, but the tank looks great, and the short plants can grow well.