I have a ah supply 4x55w
http://www.ahsupply.com/36-55w.htm setup on my 55g gives me 4wpg simple math for my simple mind. You can retrofit the kit in your existing hood. Double check before you order though. You should decide what kind of plants you want first then figure out what kind of lighting you'll need based on your plant choices. I have plants in my tank which require medium to high lighting. 2wpg would probably be where you want to shoot for as a minimum I believe but again, take your time look around and figure out your plants/style first. Good luck