Well, that's a different story. We're talking about the exposure triangle here (ISO/Shutter/Aperture), and it's always a juggling act. You have to look at it this way: You have two main objectives when shooting sports, stopping action and tack sharp photos.
For stopping action, you need to be above 1/500, period. Football isn't exactly fast-action like racing, but even at 1/500 you're going to see motion blur sometimes. If you can venture up into the 1/640 or 1/800 range and still have decent depth, then that's great. Most professional level (talking about the photographer here, not the players) football photos focus on the player and up to about a 5ft radius around them. That way you also capture a player tackling them or whatever and they are also in sharp focus. No one cares about the guy 10 yards down the field.
So, and this is just me...what I shoot...I try to keep my shutter speed as high as I can and still maintain ISO 200 (in daylight) with as high an aperture as I can. Shutter speed takes first priority, followed by aperture, then ISO. Digital cameras these days don't produce much noise, so ISO is less of a concern until higher levels.
I'm taking some football photos tonight, so I'll post them up.