Unless you go second hand, you wont be able to get a new Canon SLR like the 400D for under 650....im assuming you want digital that is......
www.digitalrev.com
this website has some of the cheapest prices ive seen and they are good i got my camera from them no hassles... i have a Nikon D70s which is fan bloody tastic for anyone starting as it can be used all the way up until your almost a professional...
the thing is, if you want to do sports photogrpahy you really need a good fast lens that is more important than the camera body.
650 is a small budget for slr sports photography, but just get something basic with maybe a twin lens kit to get you started.... if you dont mind straying away from canon give the NIkon D50 a go:
http://www.digitalrev.com/en/product...6&category_id=
thats about as good as you are going to get for that sort of price....iif you REALLY want canon, id be looking for the 350D or the 300D...preferably 350D.....