I think I've given this advice in about 3 questions in the past day:
Always keep your own priorities and goals in mind.
This new job is the one that's going to really get you on the path to accomplishing yourself. There is no question that you must accept.
You are, most certainly, screwing your new boss over, but always keep in mind that any business will promote their interests first and foremost - even if it detrimental to you!
You'll have to be as polite as possible about it, but refusing the offer you've been dreaming about should be out of the question. Simply tell your boss the truth:
I'm very sorry to put you in this situation, and I really appreciate the opportunity you gave me here, but I have just received a job offer from a company that is much closer to home, and is simply a better long term opportunity for me. I sincerely apologize for this, but I am going to accept their offer.
I would recommend leaving immediately, after all, you barely got through a single work day there. How would you give two weeks notice when you don't yet do anything there?
Good luck, and remember: always look out for number one - yourself.