What gear do you have at this point if any? That would be a good starting point. If you have some good lenses now, the cost effective thing would be to look at bodies in that line so you can save on the cost of glass.

If you are starting fresh then it is important to know how much money you want/can afford to spend. Knowing that makes it easier to get the most bang for the buck.

For sports shooting I would suggest that you say away from base line entry bodies like the XSi. One thing that is critical for fast action is auto focus speed. The Rebel series from Canon has the slowest AF speed. The XXD series has improved AF speed that is quite suitable for sports while the XD series has the fastest and most accurate AF speed. This along with some of the other features that make shooting sports easier all come at a price.

A second problem with entry level bodies like the XSi is the ISO range. It is always best to shoot with the lowest ISO possible, however most indoor events, unless held in premier venues are poorly light for photography. The XSi has a max ISO of 1600. You may find that limiting in some venues. If however you are talking about outdoor events in daylight, then the ISO issue will be less critical.

While frames per second can be helpful, the spray and pray method often of limited value. It is better to understand the sport you are shooting and learn to anticipate the action. This will allow you to be prepared to capture the shot that you want. In bull ridding some of the best shots, unfortunately for the rider, are when the rider and bull get separated. Bad news for the boyfriend, but good news for the photographer.

Glass is also going to be a critical issue. While fast glass is a blessing and a must in poor light, if you are looking at outdoor events then you can get away with slower glass. You want however good quality glass. Good quality glass usually has the advantage of quicker AF if you are taking about a line that uses in body AF motors. Some of Nikon's glass does not have this. Good glass is also considered Good because of the optical properties of the glass to provide sharp, crisp images. Good glass is going to be more expensive then consumer grade glass.

The two leaders in the sports photography field are Canon and Nikon. They have the widest range of bodies, lenses and accessories and have geared some of their bodies to this kind of photography. That does not mean that you can not shoot Pentax or Sony. Before you can do any serious looking you need to have a budget on what you can spend and then look for the best options in that budget.