"How slow" is somewhat subjective, as the acceptable amount of blur for me might be different for you. It also depends on how far your subject is from you. Because of these two factors, a specific shutter speed is not what I would recommend looking for. Instead, and since you are shooting digital - simply experiment and see what shutter speed works best for your needs.
Very generally speaking, I would start at 1/125s second shutter speed and work faster from there. You could potentially need as fast as 1/500-1/1000s to freeze very fast motion. I would say a safe bet would be 1/250s, but your results will vary. Depending on the match, some movements could be very fast and you might want every bit of the subject sharp, if that is the case you will be testing the limits of your equipment with certainty. If the routine is a bit slower or you find a bit of blur in the extremities acceptable, this should be quite achievable with your kit.
What you may find, is that you are forced to shoot at 1/125s because that also requires either a very narrow depth of field, or a very high ISO that is too noisy for your tastes. Again, personal preferences play into this.
Keep in mind also that some motion blur can be an artistic effect that adds to the image. Especially if you can shoot with a flash(but maybe you aren't allowed to), and use second-curtain sync.
Finally, why not browse through some Flickr images and examine the shutter speed used? Nothing beats real world examples to show the differences: http://www.flickr.com/search/?q=boxing
Here are some examples at varying shutter speeds. These are of a soccer game, but the same idea applies. These are all pretty poor images unfortunately.
18mm, f/3.5, 1/40sec:
50mm, f/2.8, 1/125sec:
18mm, f/3.5, 1/500sec: