Let's see... you have a 20 gal tank with insufficient light (sorry!).
First, decide if you want a low light, lower maintenance tank with plants that can tolerate it. Or if you want to go high light, CO2, fertilizer testing and dosing, and aim for a beautiful scape like Vincent Changs.
Let's say you want to go medium light, DIY CO2, and light additions of fertilizers. For a 20 gal tank, you should look for at least 2.5 Watts per gallon, which will come out to about 55W.
Coincidentially, there is a pretty good solution that you can find here http://www.ahsupply.com/36-55w.htm
for around 45$, which will get a a good reflector, ballast, and all the necessary screws and cables to get it into a hood. Then, you still need a bulb, and ppl here agree that the GE Aquarays 9325K is probably the best you can get, unless you are allergic to the slightly pink hue (your plants aren't!)
Alternatively, you could get the parts for perhaps 3 of those 24" bulbs and DIY them into your own canopy. That will give you a very even lighting, a wide variety of bulbs to choose from, and the option to "overdrive" them if you ever feel that it is not enough light.
I am not recommending the screw-ins for anything bigger than a 10 gal because of problems to evenly light a larger tank with them.