For sports action you'll want long lenses with image stabilization.
When you buy a camera you're basically buying into a lens system. That said, many pro sports photographers shoot Canon.
A great simple set up would be the Canon 50D with the Canon EF 70-200 f/2.8 IS USM telephoto lens. Make sure you'll get the IS version, not the one without it or you'll hate life.
Other long telephoto lenses are the
EF 100-400 f/4.5-5.6 L IS (comparatively slow)
EF 400 f/2.8 L IS (fixed focal length though)
EF 500 f/4 or EF 600 f/4
The lenses are all very expensive but matter more than anything else, including the difference between getting a slightly "lower end" body.
A cheaper option is to get the Canon 40D or even 30D (older) but you'll have to fork out some cash for the right lenses either way.
Now, this is all assuming you'll be doing this pretty seriously and in fairly large venues. If you're talking about taking snap shots of Junior at his local game, hell - get a Rebel XSI or Nikon D80 (either one with a good lens. Not the stock lens crap. A real lens.)
Hope this helps
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