The DSLRs are for people that want more control over their camera and can harness the ability of the lens and camera. That said, the lens that come with these cameras are slow and do not do well for stopping action or in low light. They are starter lens to get you going and find out what you want for the next lens.
What type of sports are you planning to shoot and is there a chance that it will be in the evening, indoors, or at night under light?
Your questioning on whether it's better to get a point and shoot is valid if your likely to use the auto modes. A good point and shoot can take photos that are hard to discern from a DSLR. I own both a high quality point and shoot and a DSLR. The difference is very small when shooting in normal to bright light situations.
That said, the initial purchase is just the beginning when you find out how good an external flash can be and wanting faster lens or higher quality optics. One good telephoto lens that are used in sports is double the initial cost that you're looking for in the initial purchase
I am looking to switch from point and shoot to SLR camera. I am considering the following: Canon Rebel XTI and Nikon D40 or D80. My main goal is to be able to shoot continuos action/sport shots from a distance .I am looking for a very short lag time abd start up lag time. I guess I would also need a decent lens. I heard that Nikon d40 is best for close-ups and Rebel - for action shots. Can this be substantiated or this is just personal opinion?
If I get either one - can I get by with 15-55mm lens that comes with these cameras? I heard that it is "OK" . Is it worth to get into SLR market with this kind of lens or this would be the same as high end point and shoot?