There's a recent book called Physics for Future Presidents, written by a physics professor at Berkeley, which has a lot of material on radiation risks. He says you can calculate your risk of cancer from radiation by dividing your exposure by 2500 rem. The level that gives you acute radiation sickness is 200 rem.

This is called the "linear hypothesis" and is widely used. They use it in medicine to decide if diagnostic scans are worth the risk. The risk could actually be lower; some scientists think there's a threshold below which there's no risk. There's too much statistical uncertainty at low exposure levels to know for sure.