Thinking about boosting my credit...?

I currently have an installment loan of about 5K, and one credit card. Now, I ALWAYS make sure bills come first. I am not ever tempted with mindless credit card purchases with the single card that I have. It is a Capital One card -- and they may not be the best card company -- but it is a card that boosts credit scores. I got it about a month ago. Already, I have paid it in full every time. My installment loan is always paid on time. Never missed a payment, never intend to. Now, my question is should I apply for another credit card? My reason for doing so is my knowledge of credit bureaus' tendency to give a better score to those whom can manage more things. I know that I will never max out, nor buy what I don't need. For I only use my current card for gas and GNC. Now, if I apply and I am accepted, I would only use the second card for GNC. I am asking this to all of the credit veterans out there who have a lot of experience in this. Is applying for a secondary credit card a good idea? I am 23 and I know I am young. But I know I am more responsible with money than most. I check my account balances 2-3 times a day, view statements, and check my credit score once a month. Obsessed? Dunno. Maybe just staying on top. But, anyway, what do you all think? Any help is appreciated. Thanks.