The XBox 360 does not cool itself very well. If you're going to take it apart and mod it, you probably want to increase your fan speed by connecting it to the 12v part of the motherboard so it will spin faster and cool better. But you don't need 12v all the time right? It's very loud. Maybe you want to be able to choose between the stock 5v and 12v speeds depending on what you're doing? That's the best way. This video will show you how to make a switch attached to you fan that will allow you to change it's voltage at will.