empathy vs. sympathy

Just doing some thinking about it. It seems to me that sympathy tends to derive a negative connotation... I've heard empathy being described as something that is actually dynamic, constituting of connection and understanding, whereas sympathy is seen as a passive recognition of another's situation. Is there ever such thing as too much empathy? or empathetic effort falling short in too much conscious effort to push 'past' sympathy? Is one really more 'valuable' than another? To which side is either more necessary?