I have both the 20D and 5D. WHen I shoot anything sports, the 5D is in my hand, and the 20 is in my bag, with a second shooter or with a second lens ready to go, but rarely does! The 5D is more than enough. It will definitely work.
Now, there's really no more reach with a 20 over a 5. All you are really getting is a cutout of the full image. We just had a confusing thread on this the other day! With the resolution of the 5D you won't lose anything. What you'll gain is image quality.
All things being equal, print 2 8x10's. Now, cut about 1 1/2 inches off of each side (this is in no way exact!). Now hold them side by sie. What you have is a crop where both subjects are the same size, one is just on less paper (or pixels). Just hold these cameras up to your eye and you'll see through the viewfinder what I mean. The 5D's view is much broader, that's all.
When you view the same shot from each camera on your monitor, shot's from the 20D appear closer because the monitor is filling the screen with the image. Does that make any sense?
I'll find that thread and post a link.. Someone reworded my ramblings and clarified nicely! The point is, don't look at focal length as a determining factor... There is no difference.