What plants were you looking at to put in your aquarium?
40W on a 55 gallon aquarium will allow you to grow low-light species like Java Fern, Java Moss, and Anubias. If you want more flexibility, try getting two 55W compact flourescent lights or at least three 40W normal output tubes to put over your tank.
If you want to grow a wide variety of plants you probably want something like 2W/gallon. If you go higher, you will have to look into things like fertilizers and CO2, which can get expensive.
Graeme