I have never shot any boxing myself, but I'd be wary of flash, unless it is specifically autthorized. If I was a boxer, I might be somewhat off-put at becoming the flash-ee, with possibly undesirable consequences for the flash-or. I'd suggest 1-2 cameras with fast lenses, and at least moderately fast film. See if you can find out how bright the lights are in advance.
Canon 7S, EOS 3, G10, 50D, 5D2, 7D, 95S; Leica M7; Mamiya RB67; Nikonos III. Visit jlloydphoto.com