I think metabolism is definetly an important factor in weight loss, but not the most important. I think your best bet in increasing metabolism is exercising and eating consistently healty balanced meals.
Cardio is important to burn calories and lose weight, but aslo because poor diet and lack of exercise is the second leading cause of preventable death in the U.S. Cardiovascular fitness has so many health benefits its not even funny. Exercise lowers your risk for every kind of disease and death.
Diet is an extreamly important factor in weight loss. You dont actually have to eat less to lose weight you just need to change what you eat and how often. For example I would bet that I eat way more food that a 300lb person. This is because I eat healthy food and I exercise. Healthy food has less calories than junk food. A bananna is larger than the average candy bar yet the candy bar has 3 times the calories. So by eating healthy you can eat a larger volume of food and feel full but be consuming less calories. Exercise and increases in muscle from exercising are going to increase your calorie demads creating the deficit in calories resulting in weight loss.
You don't want to try to lose weight by just altering your diet alone, and you don't want to just try to do it by exercising. Changes in diet and exercise independently will result in weight loss, but together they are much more effective and have a greater cumulative affect.
Which brings me to my next point, dont take diet pills