try Aperture Priority, look for a good high shutter speed of at least 1/640th sec and adjust ISO to acheive this, try with Evaluative metering or Canons equivilant., you may find this time of year with the days getting shorter youll struggle with a slow lens like yours.
Me againThe 40D may be the same as the 20D I had.
As best I remember Sports mode sticks the ISO on 400, the focussing on Al-Servo and shutter on Hi-Speed Continuous.
So I suspect it is using the largest aperture it can and is really Aperture Priority (with the other setting fixed).
So I would agree with Gary.
Also pray for bright days as we go in to Winter. I would also say if you are not getting a decent shutter speed (as Gary says 1/640th) tr to position yourself with players running towards you. As there is less sideways movement you stand a better a chance of a decent image (I got that advice on here a while ago - Kipax maybe).