It does really help if you know what you like to do.
I externed in a CVICU. I wanted to do open hearts, and work cardiac (hence my username). But I was bored beyond belief working there. I'm glad I externed, because I'd hate to think that I would have acccepted a job there and be stuck!
I work in MSICU, It's a combo of both and love it. We get the craziest disesases! It always keeps you on your toes.