eakers, my advice for what it's worth:
I say go for your ex's buddy if you want. It's over with your ex, and he's clearly moving on, so he has no right to any say in your dating his friend. That friend of his might LOVE to go out with you, regardless of what your ex might think. (In college, the was one friend's girlfriend I would have loved to date after they broke up, and if the situation had worked out right, would have done so regardless of what he thought).
My other bit of advice is to maybe build a new circle of friends that doesn't include your ex in it. Seeing him all the time isn't going to help you get over any bitterness you might feel, and might only make it worse.