The 20D (or the newer 30D) is a great choice, and I'll tell you why.
Unless you'll be shooting exclusively in the daytime, your main problem
will be the poorly lit fields the team plays on. This means you'll
need a lens with a large aperture (f/2.8 or at the very least, f/4) and
you'll need to use a high ISO setting. This is where the Canons excel.
No comparable model can match the Canon's high-ISO performance.
You may think you can cheat and simply use lower ISOs and a flash, but
the results won't be very good. The player will look like a deer in
the headlights and you'll get lots glowing eyeballs.
Here's a gallery of shots I took in a game last year with my 20D:
http://www.pbase.com/bret/mccallie
That particular stadium is a college field and is very well lit. Most
high school stadiums are much darker.
Here's one from a different gallery on a typical poorly lit field:
http://www.pbase.com/bret/image/48264968
Note that I had to use all the tricks for this one ... f/2.8, 1600 ISO,
and my flash as well.
If I had lesser gear I'd have to restrict my shooting to day games,
which are rare in these parts.