The first question I would ask, since you state you will be taking "lots" of boxing photos is, "Are you doing this for fun or profit?" Are you photographing for money, a friend who is a boxer or just because you are drawn to the field?
If you are doing this for money, my first suggestion is that I would question your ablility to provide your client with a quality product, seeing that you have to ask what lens to use. Secondly, if for money and you think you can do the job, what about renting a MkIII or MkIV and a couple of lenses to do the jobs. In the short run, it will definitely save you money instead of buying the wrong lens. It's a great way to experiment with different equipment than what you own. Later, when you know exactly what YOU want, you can buy the right equipment.
If this is for fun, well then...enjoy. In that case I would go to a few fights and see what kind of quality shots you are getting. If you find your talent is wanting, why don't you look at SnapLocally's school?
If you've never photographed boxing before, excuse me for making an assumption but that's what it sounds like, then you have no idea how fast the action is or how low the light usually is.
Having NIkon myself, I really don't know what Canon lenses I would suggest, in the Nikon line, my boxing wish list would be a couple of d3s bodies (10fps and really great hi-iso), a 14-24 2.8, a 24-70 2.8 and just for the funky shots inside the ring after the fight is over, a 10.5 fisheye.
Good luck, come back on after a few fights and let us see what you get
Jerry