http://www.americanaquariumproducts.com/AquariumUVSterilization.html
Read this and the associated articles, they helped me a TON. Really, at the end of the day, it depends on what you want out of UV and how much you are willing to pay. You could get something thats pretty darn cheap and have it kill green water for you, or you could get something more expensive that can kill bacteria and even parasites.
I'm running the Vecton 600 on my 75 now, and I super recommend it to anyone who is thinking of going into UV. My fish have been looking a lot healthier and been more active ever since I added it, and my algae has been cut down significantly (it won't do anything for algae that is already in the tank such as green spot algae or BBA since those don't really move around, but it will kill them when they get into the water column).