First off, I don't shoot much in he way of outdoor sports, other than a little motorsports, and I certainly don't use flash for that, so the following is all just from my head rather than from any practical experience. I also see you're a Nikon guy and I'm not sure about the ins and outs of Nikon's offerings, but, being a Canon shooter myself, here are some thoughts....
- On cameras with a builtin flash the Guide Number of the flash is 13m. The guide number of the 580EX flashgun is 58m, or over 4X the distance. So straight away there is a big advantage in reach with an external flash.
- With a 580EX you can use a device called a
Better Beamer to further increase the reach of your flash when shooting with long lenses.
- You may have a problem with use of flash limiting your sync speed, and making it completely unuseable for freezing action in conditions of moderately bright ambient light. e.g. the sync speed for Canon builtin flash is 1/250. With a 580EX you can use high speed sync, allowing flash at up to 1/8000 but at a very reduced power.
I'm sure the constraints are very similar in the Nikon world. So the questions to be answered are....
- What is your subject distance?
- What lens will you be using (focal length and max aperture)?
- What aperture will you be using for the required DOF?
- What will the ambient light levels be like?
- What is the maximum ISO you are happy to use?
- What minimum shutter speed do you require for the sport in question?
- How many FPS do you want to shoot at?
- Are you intending to use the flash to fill shadows (backlit subject) or as a main light?
- Will flash use be allowed at all by the event organisers?
If you can provide answers to the above I may be able to offer a better response but my guess is that you'll either be unable to use flash at all or will need a serious flashgun to make it worthwhile. If you do use flash it may limit your shooting speed significantly. I suspect (not having done it myself) that for outdoor sports a builtin flash is likely to be as much use as a chocolate teapot (probably).