Dual cores hold the game back a bit, but you should be performing better than that. What are the rest of your system specs? I'm particularly interested in how much ram you have.
For reference I'm running the game with a Phenom II 965(stock or overclocked doesn't effect my fps), 8Gb DDR3, and a 5770. I manage an average of 45fps mostly medium settings 1920x1080.