Hi Christian,
The general rule for shutterspeed is to use at least the same shutterspeed as the focal length. This means:
1/100 sec shutterspeed for a 100mm lens
1/250 sec shutterspeed for a 250mm lens
1/400 sec shutterspeed for a 400mm lens
These are all minimum shutterspeeds. In Europe it's not uncommon to find the correct camera setting at ISO 100 - 1/500 sec - f5.6. But with todays sensors it should be absolutely no problem to increase ISO to 200 or even 400. This would allow you to use f8 most of the time. I know that with the EOS 400D you may want to avoid ISO 400, but ISO 200 should be no problem at all.
Regarding my own settings for action photography, it depends on the situation. Sometimes I drop the shutterspeed to 1/125 or even 1/60 at my 300mm when I try to achieve background-, propellor- or rotorblur. But when intentional blur is not an issue, I do tend to use 1/1000 or higher for shutterspeed. For me apperture is not as much an issue as shutterspeed is. After all, unsharpness caused by camera motion is a bigger problem than unsharpness caused by lack of depth of field. And as I use a camera with a 1.3 crop factor, I don't have to worry too much about vignetting as dark(er) corners stay outside sensor view. Your situation with the 1.6 cropfactor is even better.
But when you have to shoot near apperture minimum (f5.6 with your lens), apperture will become more of an issue than with my lenses (most are f2.
![Cool 8)](/forum/images/smilies/icon_cool.gif)
. So you will have to make your own decision on which settings to choose. There are many factors to consider.
Hope this helps.
Cheers,
Iwan