Buddhism takes the middle path; that's why there's no hell or heaven but in between. What I've heard is that we've to get birth again n' again coz' we carry the sins. Sins are the seeds for re-birth. As soon as we can purify ourselves i.e. by getting enlightenment or "ARHAT" or "NIRVAN", then no more birth. "Bodhisatta" has many lives before he can be "BUDDHA". But there're many examples in buddhist text of getting "Arhat" in one life.