I have a 20-gallon TALL tank that I am going to plant. I plan on getting a
light kit from AH Supply, but I'm not sure what I should get to light this
tank. I know I'm gonna need a lot of light since the tank is so deep, but
beyond that, I'm clueless as to how much light I'll need. Any suggestions?

Go with the 55W if you want a thick foreground, otherwise the 40W would be
better. I'm running the 1x55W over a 15gal, its really strong light.... I
would highly recommend a compressed CO2 system; I was unable to keep things
under control with DIY CO2 and the 55W.... 5000 or 6400k, stay away from
10,000k bulbs and actinics

Cheers,
Jeff Ludwig