if I were you I'd look up Canon 10Ds or 20Ds, might be able to find a good deal.
I am a professional photographer and just upgraded to a Canon 5DMkII from a 40D. 70-200 2.8 may be a bit much $ for someone who isn't even a hobbyist, but assuming your ultimate frisbee games are during the day, that much firepower is probably unnecessary. A workable so-so quality 300mm zoom can be picked up for around 100usd on ebay. Kit lenses will have enough macro (combined with megapixels) to get good pics of fish.