"Talk it over" idiomatically means to discuss thoroughly. Talking "about" something can mean this, but not so specifically. "Talk over" also suggests that you talked with a view to achieving an outcome, or a decision. "Talk about" covers more general, casual conversation.
In your example, either work fine - because you have stated a topic for discussion (your sales idea) and who you are discussing it with (your boss) it is pretty clear that you intend to achieve something as it is a work-related discussion. Still, "talk over" does sound like you were more thorough and achieved something at the end of it rather than just a preliminary discussion.