this would all depend, what size tank do you have? How high is it?
if you have an eclipse tank it woudl cost a hell of a lot because you owuld have to rebuy everything except the tank and heater. But for theses, and your normal hoods, there are retro-fit kits that let you instal either PC or VHO yourself.
Or you cau buy a whole new lighting setup and take out your hood. Unfortunatly, this site does not have lighting, so i recomend serching for retrofit kits if you want to keep your hood, or either PC or VHO lighting. Even aquarium lighting. The famous auction site, you can get some good deals on lighting.
Suposedly the new jebo/odyssea lights are good, and are pretty inexpensive too. I jsut ordered some the other day so they should be coming this week.