Is this enough light???

Cost effective really depends on how much you can do yourself. Are you looking for something ready made or something you can build yourself?

If I was in your situation and didn't want to replace my full hood, I'd gut the 24" fixtures and run two incandescent bulb fixtures into each 24" shell. I'd then put 2 daylight spiral fluorescents in each. They are 5-10 dollars a piece and I've seen them up to 30 watts. This would give a possibility of around 2 wpg and keep your current hood.

If you're handy, you could build a wooden box and put 2 shop lights in it. This would give you about 3 wpg.

Before you change anything, you could try some low light plants and see how they do.
http://www.aquariumplants.com/Low_Light_Plants_s/25.htm

you should be shooting for 3 to 4+ watts per gallon, as a generalization, for growing plants

:silly:You can do low light plants with 1-2 and medium with 2-3. 3-4 would only be required for plants with high light requirements and when you get to that point you're looking at adding CO2 and fertilizer.

Your best bet is to head to www.ahsupply.com and pick up a couple of 55 watt kits. They will fit in your current fixtures if you mount the ballast on back. And they will get more usable light into the tank than any other light you can put there.

Spend your money right the first time. Don't mess around with half measures. That just wastes money.

With 2 55 watt AH Supply kits over a 55 gallon tank you can grow most plants.