Yes, retake it, especially considering your undergrad grades. Those combined with your bio score indicate a potential weakness in the sciences, and the first few years of dental school have some brutal science courses. Use Kaplan's material for the bio section; buy their subject tests if you have to and use the questions as well as the explanations. There's a ton of good info in the explanations, and if you're thorough, it'll beef up your bio score. Your GC and OC scores indicate that you can definitely increase your bio score.