The best place is generally somewhere near the center of the area where you want wireless service. Ideally, the signal should pass through as few walls as possible, as it travels from the router to your wireless computers. Try to locate it away from other electronics (i.e. don't put it behind your monitor, near a 2.4 GHz telephone, or near a microwave.). If you have a primary desktop computer, there are some advantages to keeping it within eyesight, as this lets you quickly check the status of the router if there is a problem. Finding a good location is sometimes just a matter of trial and error, so don't hesitate to try out a few spots. If you find 2 locations that are equally satisfactory to you, then consider your neighbors, and locate it where it is least likely to bother them.
I don't think you will have a problem wherever it is placed. I live in a two story house and have good reception anywhere inside a 500 foot radius. The simple location is where your internet connection is now with that PC wired to the router. All other connections can be wireless but you really need one that is direct.