From the SRD:
Bull Rush Results: If you beat the defender’s Strength check result, you push him back 5 feet. If you wish to move with the defender, you can push him back an additional 5 feet for each 5 points by which your check result is greater than the defender’s check result. You can’t, however, exceed your normal movement limit. (Note: The defender provokes attacks of opportunity if he is moved. So do you, if you move with him. The two of you do not provoke attacks of opportunity from each other, however.)