do you mean a CFL forty watt equivalent? o_O
if it's incandescent, get a CFL instead because incandescents make too much heat...
40 watt equivalent would work well though. you have good low light plants.
I had a 5.5gal with a 26watt cfl and I had to downgrade to a 13watt b/c it was too much light, I even raised like 6" above the tank and it was still growing algae. No problems with the 13 though, and I am able to grow high-light plants.