Idling at 40 Celsius is fine... But you live in Michigan, right? Depending on where you are in your house, this can be fine or bad. What you really need to watch out for is your load temperature. Idle temperatures are really no big deal; they are almost always fine unless something is wrong with your cooling system. Load temperatures is what you should watch for. Like Dev said, 55 Celsius is a good limit. While AMD's can take temperatures up to around 60-65 Celsius, going over 55 Celsius isn't too great.
Personally, since I upgrade a lot, and I probably will be in a year or two, I'm not so concerned on temperatures, since a little warm temperature is not going to reduce the life of a component that much if it is still in the "okay" zone. My load temperature is about 50 Celsius, more or less. It has jumped to 52, once.