Motorola SURFboard SB5100 Cable Modem Multiple Users Problem

I currently have my Motorola modem connected to our dynex wireless enhanced g router to supply the internet to our house for the computer (mac) and my ipod touch. The problem is i also c dynex onnect my xbox 360 to our router (by wire). Whenever we try to use 2 devices at the same time the internet runs extremely slow to where you can basically not use 2 devices at the same time. Please tell me if i need anther set up or a better connection. Any help at all would be appreciated -thanks