At temps in the 50s, a beanie should be adequate. If the rest of your body is cold, you might want to invest in a long-sleeve jersey, wind vest, long-fingered gloves and wool socks. Knee and arm warmers are an option too and easy to shed if you get too warm. Don't worry if you are a bit on the cool side when you start out. After your warm up you should aim for keeping the temps just comfortable and minimizing perspiration. While you may be used to riding soaking in sweat in the summer, in cool weather, drier is better. Remember that perspiration does not directly correlate to effort or calories burned. You can sweat profusely reclining in a sauna or burn a huge amount of calories shivering in a cold wind. Layering with 1/2, 3/4 or full zippers in the front give you excellent thermoregulation. Just adjust to the cool but comfortable even at your peak efforts. If I come back from a winter ride with my innermost layer just damp in spots, not soaking, and haven't experienced any hard shivering, I feel I did it right.