Brooke, I've been told that the heart rate is only an "indicator" at the end of pregnancy, after 36 weeks. Most babies heart rate's are faster at the start and gradually slow down as the baby grows. I think there may be some truth to it but obviously only in some cases.
I'm like you at the moment, dying to know the sex of my baby, but I'm trying to be patient and not dwell on it too much. 8-[