The depends, 2 40w tubes over a 55 isn't bad for a low light setup. How many cfl's could you put in that hood? What wattage tube can you use in the back? Keep in my, the highest wattage CFL's on the market are 26w. If the hood you are talking about only takes two, that's 52 watts, but since CFL's are only about 86% efficient, that's now down to 44 watts. If the back bulb is only a 40watt tube, added with the two CFLs, that's only 84 watts, same as two tubes, and you have the option of putting 10,000k bulbs in if you like....and option not available with CFLs.
Of course, if you can put 4 CFL's, for 144 watts, factor in the inefficiency plus a 40 watt tube...now you've got 130w of useable light over that 55g, and that is a perfect amount of light.