Damage/Speed Cheats 
     These  codes  increase  punch damage and speed. Enter them in the
main menu.

Damage - hold R, press RIGHT, DOWN, LEFT, UP, LEFT, L. 
Speed - hold R, press DOWN, LEFT, DOWN, UP, RIGHT, L. 

Max Stats 
     To  even  the playing field and have max stats for all boxers, go
to  the  main  menu  and enter this code: LEFT, UP, UP, DOWN, RIGHT, L
while holding the R button (thanks to Mike).

Max Stats (Movie Mode) 
     Screw  the  work  out.  On the main menu hold R then press RIGHT,
DOWN, DOWN, UP, LEFT, L.

Unlock All Boxers And Arenas 
     Unlock  all  the boxers, statues, and arenas by doing this at the
main  menu: hold R, press UP, DOWN, DOWN, LEFT, LEFT, L. This code and
the other Unlock All Boxers on IGN are mutually exclusive!

Win A Fight Cheat 
     On the main menu, hold R then press RIGHT, RIGHT, LEFT, LEFT, UP,
L.  While fighting in movie mode, get an instant win by pressing BLACK
+ WHITE.

Unlock Mickey Goldmill 
     Beat Movie Mode while playing through on the Champ setting.
 
Unlock Most Boxers and Arenas 
     To  unlock  all  boxers  (except Rocky Statue and Mickey) and all
arenas,  go to the main menu and press these buttons: RIGHT, DOWN, UP,
LEFT, UP, L while holding the R button (thanks to Mike).
 
Unlock The Gold Class Tournament 
     Beat the Silver version of the Knockout Tournament.
 
Unlock The Rocky Statue 
     Beat Movie Mode while playing through on the Contender setting.
 
Unlock The Silver Class Tournament 
     Beat the Bronze version of the Knockout Tournament.