Any of those lenses should work....even if you only had a 50mm you could get decent shots.
But I'd say go with the 28-120mm...one minute they may be right in front of you and then 10 seconds later they are clear across the ring.
I would go with a manual setting.
Get there early or before the bouts you are shooting and use a hand held meter to measure the light. Measure all around the ring and attempt to get in the ring to measure the light in the center.
If there is TV coverage then the light should be pretty good. You might get 1/500th of a second at f2.8 at ISO 800. If there is no TV then the light may be very poor.
You may get 1/250th of a second at f2.8 at 1600 or worse.
At a 250th of a second you will get some motion when they throw punches.
Whatever readings you get I'd say dial them in on a manual setting and leave it there. You don't want to be thinking about exposure....you want to concentrate on the action. The light isn't going to change.
Also there are good pictures to be made between rounds and you might get some good shots also when the decision is announced.....look for jubilation from the winner and dejection from the loser.
Good luck! [img]/ubbthreads/images/graemlins/cool.gif[/img]