I retrofitted my 29g.
I opened up the existing light fixture as shown in that link and ripped out the original wiring completely. I picked up a Coralife 65w PC ballast kit and a reflector and just popped it in. With that particular kit, there's really only room for one light in the existing cover, and with a reflector it's a snug fit. I'll try to post a pic later.
The lighting is pretty limited to towards the front of the tank, which kinda sucks. Eventually, I'll probably rip out the guts of the hood completely and just use it for aesthetics, and then put in a glass cover and position the light properly... That also means replacing the filter, but I've been getting flack from a few people here to upgrade to a canister anyway. Maybe soon... maybe very soon...
29g Eheim Classic 2213, community, heavily planted, 65W CF, 2x Hagen CO2
20g Fluval 3 Plus, nothin' but shrimp & snails, 65W CF, Hagen CO2