I used a Canon digital rebel XT for our entire football season. Managed to amass about 3,000 pictures that I could classify as "keepers".
I used a Sigma 70-200 zoom 2.8f lens and operated the camera in the "creative zone". Generally I would be able to shoot at about 250/th shutter speed but it generally depended on where I was positioned and what angle I was shooting onto the field (if the shots were coming from the stands and hitting the center area of the field I would get more light and get a faster shutter speed, if I was on the field and shooting across, the shutter speeds would slow down as the amount of light was diminished some).
I would generally shoot something between 150-250 shots a game, come home and scroll through them all, discarding what hadnt gotten enough light and was blurry, or just in general were "bad" shots. I recommend getting a 2gb flash card and just going nuts when taking pictures. You will find some amazing shots in the mix and will discard those that don't do well.
I then utilized walgreens.com and posted every "keeper" for our entire school to pull from. All it took was to make sure that everyone had the login and password and they could go and select the ones they wanted, choose the walgreens they wanted them from and then just go pick them up the next day.
The biggest consideration when buying a camera is not just the body, in fact if I weighted the importance of body vs. lens, I'd say the lens is the most important. Without a low f stop. The lower the number the better. You'll end up spending 1200-1500 for a canon lens and about 700-900 for the sigma equivalent. Both lenses are great lenses for football at night.
Sorry for the lengthy post, but I hope you find it helpful. Have fun, it's an addicting thing once you get into it.