You might want to look at these for input on shutter speeds:http://www.pbase.com/rhowland/2005_06_12_watkins_glen
The cars were about 200 yards away, moving across my field of view at 80-100 mph. The lens was my 70-200 Sigma EX. The longest lens I've ever used for panning was a 100-400.
I've never been able to use a tripod or monopod when panning. When you're panning, you have to rotate your body not a tripod or monopod. I have found that I want my body in its most comfortable resting position at the end of the pan. Then twist your body, with your feed stationary, to where you expect the pan to begin, wait for the car to enter the viewfinder and unwind your body while following the car. If you're most comfortable at the start of the pan, you'll tend to lag behind the car. With my 40D, I was shooting at 6.5 FPS, which mostly was adequate. The 10D used for these shots was a bit slow. I wanted the car to be directly perpendicular to me or with just a little of the front showing, so timing was extremely tight. I'm waiting for Canon to introduce a mirrorless FF camera that can take 20 FPS, full resolution.