I've shot a few charity boxing matches. From my experience you need nowhere near 1/400 of a second. Obviously your camera can take it though with regards to bumping the iso where as mine at the time couldn't. I used flash at all of them and just shot straight up with it. Firing it directly at them is not only bad for the boxers but the results usually look awful aswell.
Remember that any spectators won't be best pleased with you standing up all of time in front of them so expect a little agro from them, especially later on into the night if there is alcohol involved. Pick a neutral corner and preferably one which has the least spectators around it.
I found picking my shots and getting about 10 action shots a round was more than enough. Then just keep your camera ready at the intervals inbetween rounds for any good shots that may present themselves such as the corner man misting the boxer with a sprayer. I used to like to get creative with the lighting if i could but all that takes time and isn't worth worrying about as you could miss a vital shot whilst fannying around lining the light up with a boxer for some rim lighting or silhouette shots.
Most of all though mate, is to remember to enjoy the occasion and experience!
Good luck and don't forget to share a few pics afterwards!
David.