I shoot boxing a bit, when shooting ringside I use two lenses mainly. A 24-70 f2.8L and a 70-200 f2.8L, (sometimes I'll mix in 16-35, but I mainly use that as an overhead remote lens) both on 1D Mark IIN cameras. As far as what makes a good shot, I advise to shoot tight, legs are usually meaningless in an action shot. However on a knockdown, you'd like to get wide to get in both fighters and the referee because that is what tells the story.
Set a white balance, shoot manual, a good starting point is somewhere in the f2.8 1/500th iso 1000 range and adjust from there...I tend to manual focus alot, but when autofocusing set a point to the left or right of the frame, center point wont help...
Try to avoid getting ropes in the shots, bring tape (black) to tape the ropes tighter and avoid getting colors showing up in the shots. I also have red, blue and white tape in my bag as well, just in case TV complains about black tape, doesnt help with the colors in the frame at times, but does tighten the ropes...
You can use the ropes to shield the camera a bit from lens flare shooting into the lights. I also remove any filters from my camera too. (I find it helps). Bring a cloth to wipe down lenses and cameras from blood and sweat, it's usually flying all around...
In between rounds, dont forget the corners and crowd, lots of good shots happen in between rounds. Good luck...
Here is a link to a gallery that will show you examples of tight action shots vs wide knockdown shots and a few others mixed in...
http://www.sportsshooter.com/muls/boxing/index.html
Just a note, jpeg large is fine, I only shoot RAW in certain circumstances and even then will shoot both, but HBO, ESPN and others all request jpegs while on assignment, particularly on deadline work RAW is time consuming.
Ed