Pallas (41, 104): This is a reference to the Greek goddess Athena, often called Pallas Athena, or just simply Pallas. She is primarily associated with wisdom, which makes her head an ironic place for the Raven to sit, since we can never quite tell if the bird is actually wise or is just saying the only word it knows. Since she's a goddess, though, she's also a symbol of the ideal woman, perfectly beautiful wise, virtuous, and strong. For a man who spends all his time thinking about the perfect maiden he has lost (Lenore), a bust of Pallas seems like a pretty good choice.
Balm in Gilead (89): This refers to a biblical quote, from Jeremiah 8:22 "Is there no balm in Gilead; is there no physician there?" In a general sense this famous balm (a kind of healing ointment) has come to represent hope, peace, an end to pain. Obviously, since the origin is Biblical, there's an aspect of the peace of Christian salvation, although we can't quite tell how much the speaker of the poem believes in that.