I'd recommend shooting in shutter priority at 1/125 or slower if your panning ability is top notch.
Otherwise, the cars are going to look like they're parked on the track.
If you're shooting the cars head on then it won't make a difference so it depends on where you're shooting in regards to...
Are you shooting Tv and AI Servo? Do you use a monopod or tripod?
1. utilize the 40d's 6.5 fps and shoot 1/500 or faster.
2. Save up for the 70-200mm f/2.8
3. ISO of 800 or less with the 40d, nothing higher.