The bust of Pallas is a short way of referencing the Greek goddess Pallas Athena, or rather, Athena-- the namesake of Athens, and Greek goddess of wisdom.
It symbolizes that what the raven says should be considered as truthful and wise.
Poe said he chose the word "pallas" due to the sonorousness of the word..the way it sounds. "Bust of Pallas" fits the meter better than "Bust of Athena" but you can't believe for a second that the choice of Athena, stated as Pallas, was a randomly chosen Greek deity that just sounded pleasant.