You should shoot for 2 watts per gallon for a low light setup. You only have .67 Watts per gallon. If you want to try co2 injection & nutrient addition (more of a hi-tech method), considering the height of your tank, you should try for 3 watts per gallon.
I would first try the low light approach. You've got to add at least one more 40 Watt bulb for a total of 80 Watts. I would try Java fern, various Anubias, and Crypts, because of your low light, as these plants generally tolerate low light pretty well, and are generally universally available.