Interview -> Job offer

So.. I had my first RN job interview and it was terrifying!! It was a panel interview with the managers from all departments and me at the head of the table. They asked me questions like, "Explain a time when.." followed by scenario questions. It wasn't the most difficult interview but I felt very well prepared. 2 days after the interview (today) I was called and offered the position!! This is the hospital where I already work so I had a good idea that I was going to be offered a position. However, I did have to jump through all the hoops like everyone else.

Here is my dilemma. My dream job is at our sister hospital in their ICU. I REALLY want this position because they have cardiac services and I ABSOLUTELY LOVE cardiac. I really feel like this other hospital is advancing in their nursing care and actually utilizing evidenced based practice and new research. They actually started doing therapeutic hypothermia!! Now for those of you who have more advanced hospitals this may not be a big deal, but to us here, this is something we only read about!! I feel that if I was offered a position at my dream hospital then I would have the best opportunity to become the kind of ICU nurse I dream of being.

Heres the problem, nursing jobs are not easy to come by. It is an absolute honor and accomplishment to be offered a job by a department I am interested in before I graduate. I understand this. But the question is, do I keep working towards a position of my dream even though I accepted a position at my hospital? The hospital which offered me a position told us that they understand if we take another position since they are offering jobs so early in the game so I know it wouldn't be frowned upon, but how do I explain to my boss that I really dream of working in a cardiac hospital?