Last football/soccer season I shot a Nikon D3, ISO 6400-10,000, everything shot RAW and processed in Photoshop CS3, and using high-end glass, be it 80-200 f/2.8 or same lens with added 1.4x adapter (for comparable 300 f/4 lens).
Lights varies greatly between fields, too. Sometimes even in-field light levels are all over the place.
Glass is the biggest requirement, something in the f/4 or f/2.8 range, and the darker it is, the higher your ISO or shutter needs to be.
High shutter speeds de-blur the image, stops motion more. At least 1/200 or more. It really depends some on the angle and distance too, as to how high is required to stop motion.
A speedlight flash helps to stop motion, but you should not rely on it entirely or even as much as 25% of the light. Top of the line too, not the low-budget weak ones.
And please take it out of "P" mode or "Sports" mode -- learn to shoot this manually. It's one of the few times I still have to shoot manual, outside of a studio, for the best quality. Cannot rely on semi-auto or full-auto programming.