Decide what kind of plants you'd like to keep and what kind of work you want to put into the tank before you make a purchase. With more light comes more ferts, CO2 in higher light, and faster growing plants as well as algae.
If you are just wanting to increase your plant choices a bit and not get into CO2 just yet, a 130w fixture will work. Even though your 55g has 2 fixtures now, a 48" 2x65w PC fixture should fit. 130w over the tank, if you have alot of plants, shouldn't make you need CO2. I had that over my tank for awhile without CO2 and just had minimal algae problems. My tank is packed full of plants, mainly stems though.
A 260w PC fixture will allow you to grow just about anything, but you'll need a fert schedule and CO2 (pressurized) with that fixture.
There are some T-5 fixtures that might work, but the wattage won't be much more than you have now, unless you get a fixture that is meant for saltwater and replace the bulbs.
You may be able to retrofit 2 65w kits into your existing hoods. Check out
http://www.ahsupply.com. I can't help ya with that though because I'm not electrically inclined.
