I've been taking a lot of rugby lately using a 70-200 f2.8 have a look here
http://www.flickr.com/photos/rkspics/
You have to know what the highest ISO setting you can use that still gives a clean enough image and that will let you get a reasonable speed to freeze the action. Have a look at images on flickr and you will see a link to more info, read that to see what ISO and aperture they are using to get a given speed, pray that your games are on bright sunny days to get the spped up and ISO down.
I never use flash, the players will linch you for blinding them and you will quickly leave the event.