I have the Canon XTi, using the zoom lens EF 75-300mm. What is the best setting for outdoor night sports photos (with stadium lighting)
Years ago I tried to shoot night sports with an XTi and f/4-5.6 lenses. Unless the artificial lighting is equivalent to a pro level stadium, you're probably not going to be able to get what you want. Even if you are going to push the ISO all the way to the XTi's highest setting of ISO 1600, your required shutter times to freeze the action will be out of reach with an f/4-5.6 lens. I regularly shoot at ISO 3200-5000 to get shutter speeds of around 1/500 second with f/2.8 lenses in local sports stadiums where I live.
What you can try is to set your ISO all the way to 1600, set your aperture wide open in Av mode, and dial in about -1 exposure compensation (most of the background is dark and you want it to be dark, not medium gray). You'll then have to live with whatever shutter speed you get. Note than when zoomed out to 75mm your lens is at f/4 and will allow a shutter speed 1/2 as long as when your lens is zoomed in to 300mm and at f/5.6.
Using manual mode is helpful when shooting in these circumstances. This is due to how the camera meters the amount of light in the picture. The camera doesn't necessarily know the difference between a dark shadow and a dark object. If your subjects are dark (football players wearing black uniforms) then the camera is going to try and slow the shutter down to compensate, and increase your risk of blur.
I would recommend the highest ISO that doesn't create an intolerable amount of image noise, the largest aperture (smallest F-number, F4 @75mm in your case), and then the fastest shutter speed that captures enough detail of your subject.