How much lighting is too much?

I have been keeping plants for a little over a year, but would say I am
still a beginner at this. I am going to upgrade the lighting on my 46
gallon bowfront tank from a single flourescent, giving me right now ~1
watt/gallon, to a compact flourescent fixture. I am looking at the Coralife
deluxe series and am debatting between the 96 watt fixture, giving me 2
watts/gallon, and the 192 watt fixture, giving me 4 watts/gallon. It is a
36" hood. I do not currently have any supplemental CO2. I am leaning
toward the 192 watt, but am wondering if that will require CO2 to keep
things in check. It would be nice to be able to keep some bright light
plants! Any advice?