First, it's not true that cardio 'burns muscle'. Cardio, like any exercise, burns calories (which are a measure of energy). Your muscles, as with any part of your body, require energy for maintenance. If you do not have enough energy to maintain your muscles, they will waste away. The problem is therefore not the cardio, but how much you eat. If, after you have done cardio, you still have had enough calories to maintain your muscle mass, that's exactly what you will do.
Cardio is considered useful for losing fat for exactly the same reason - it burns calories that may be needed to maintain your fat levels. Don't think of cardio as burning either muscle or fat; merely think of it as using up your energy. What your remaining energy does depends on the quality of your diet and how much muscle your body thinks it needs (ie through a weights program). Those two facets are FAR more important to get under control.
This guy does cardio:
http://www.crossoverentertainment.com/i ... %20Out.jpg
This guy doesn't do cardio:
http://file001.esportsea.com/images/teams/44487.jpg
Whether or not you need cardio will depend on your metabolism, your diet, your weights program and your body fat levels. I won't pretend to be knowledgeable enough to tell you if you should do cardio or not; what I will say is that before considering adding it to your workout, you should be fixing up your diet and calorie intake, because that's the issue you're really trying to address (calorie usage) in an indirect way.
Your metabolism will never be good enough to get ripped without a fairly strict diet and a good gym regimen, I'm afraid to say. You will get a metabolism boost, but it won't be nearly enough.