The wattage rating of a bulb is linked to it's length, which is why you can't (and won't) find any 18" bulbs greater than 15 watts. To get more wpg you will have to add to or replace your light fixture. 2 or 3 wpg is actually a pretty significant difference and you'll want to decide now which you want to shoot for. 2 wpg will allow you to grow most plants, except for high light plants, won't require CO2, and might require some ferts. 3 wpg will almost definitely require CO2 and ferts, but you'll be able to grow almost any plant out there.
For 2 wpg on a 55, a 2x65 watt compact fluorescent fixture would be perfect, and these come in 48" lengths.
BTW, I have a 55 with 2 wpg and no CO2, and I just posted a pic of it in the planted aquascaping forum.