Thanks everyone for your thoughts and suggestions, they have been very helpful (I hope so).
I've spent a second day in the gym, with the guys actually training with boxing gloves and helmet. First thing I have to say is that it has been a very funny and interesting experience, which, I hope, I will be able to preserve in the future. I don't have any particular techincal skill so I hope that this, among other things, will help me grow in taking pictures.
The gym is located in a suburbian part of my town - somehow, it is a safe place in what the media calls a degraded neighbourhood. I really liked the "real" nature of the people there, and also the fact that they appreciated my presence there - I tried to be unobtrusive and let the guys act naturally.
Now on to the pics. The main difficult, using a manual focus camera, was getting shots of people in motion. Indeed, I saved 25% of the shots I took. I also tried to get different point of views, even entering the ring myself, laying down and shooting the guys from the bottom.
As I said, i actually saved a small part of the work; some pics taken with the boxeurs moving and training, plus some portraits and poses. I will post here just a sequence, if anyone wants to see the full set here it is:
http://www.flickr.com/photos/nicola...6268169255/in/set-72157627949601650/lightbox/
And here is the sequence: