I think it depends on what you're shooting. When I started shooting motorcycle roadracing, I bought a 300mm f/4 because it was the minimum focal length that really allowed me to get tight on my subjects. And because motorcycle roadracing usually takes place in midday sun, a really fast aperture isn't that necessary. Furthermore, the 300mm f.4 was the best lens I could afford at the time

So why don't you tell us more about what you shoot and what your budget is like. You might also want to take a look at our Sports Photography Forum and see if you can find anything useful there. It's a really good source for action shooting information.