My A700 is set at ISO 400 most of the time. But a lot of the time I have a tele attached. If the light is good and I am shooting stationary subjects (either with tele or the CZ1680) then it is set at ISO 200. Last year, to shoot lightning where I was looking for long exposures I used ISO 100.
Probably 95 percent of my images are shot at ISO 400.
You'll probably want ISO 400+ for your motorsports...most (most) of the time you will never see noise. On gray days you will pick up noise in clouds and dull parts at ISO 400. Interesting....if you really want fast shutter speeds use ISO 800 when the sun is shining.
In good light you will see virtually no noise at ISO 800.
I agree with tim's findings. Digital ISO 400 is way cleaner than film ISO 400. We are sooooooooooo lucky to have decent images at such high ISOs ... we are doing things we could not even dream of 20 years ago. How cool is that?
Kodachrome 25 or Kodachrome 64 anyone?