The basic guideline for minimum shutter speed when hand holding is usually....
Min SS = 1 / (focal length x crop factor)
so for a 400mm lens on a 50D you should be looking to use a shutter speed of at least 1/640 if you have steady hands and a stable stance. However, that guideline is for image enlargements of up to around 10x8 or so. When you view a 50D file at 100% it is like viewing a virtual image which in total is more like 39x26, or pretty much 4X larger. Thus for the image to bear scrutiny at 100% magnification you should be looking to use a shutter speed of 1 / (640 X 4) = 1/2500. That is for shooting from a steady position. If you are tracking a moving target you may need the shutter speed to be a little higher than that.
Of course, if you can fill the frame and do not need to crop, and only print//display at more conventional sizes then you can relax and go back to the 1/640 minimum, plus a bit for safety, so maybe 1/1000 as a realistic minimum, but don't expect the pixels to look sharp at 100% if you shoot like that. You may get lucky but don't count on it.
For these reasons I question the value of high pixel density sensors when shooting action. They may be great for yielding lots of detail when you have stable support and can control both camera movement and subject movement, but for action shooting you will need a fast shutter speed if you want to make the pixels count and in anything but superb light you will need to bump up the ISO, thus increasing pixel noise and limiting the usefulness of individual pixels. In my opinion my 1D3 has just as much useful reach as my 50D when it comes to shooting action as the 1D3's high quality pixels can be used at 100% far more readily than my 50D's noisey little blurred pixels. Shoot from a tripod on a slow or static subject and the tables are turned in favour of the 50D, if I can keep the ISO low enough (100 or 200).
In full sunshine at f/5.6 you could shoot at 1/3200 and 400 ISO, which isn't too bad at all, but 50D pixels do already look noisey at 400 ISO so don't expect to crop too aggressively and retain detail. However, if the action isn't too lively you might be able to drop the shutter speed to 1/1600 and the ISO back to 200, which would be pretty sweet.
The most important thing is to understand the mathematics of all this and not to beat yourself up, or your equipment if things look a little fuzzy at 100%. At 33% they will probably look great.