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I think a good rule for action sport photography is to start with the shutterspeed and then adjust aperture and ISO accordingly. I read somewhere that in order to freeze soccer action with kids, you would need a shutterspeed of at least 1/400. At those speeds, and with your 85mm lens you would not need a tripod, that would only limit your movement.
I was going to suggest the same Americo (I hope I am remembering your name properly). I tried something new over the weekend (after talking to the folks here) and I set my aperture and speed manually and let the camera pick ISO. My early results are quite positive and would suggest the same to you. Since I was in bright sunlight and wanted to eliminate any shake induced blur with the focal length I was using (500/8), I set my min shutter speed to 1/800 (in the ISO settings), set my camera to 1/1000, aperture is fixed at f/8, and let the camera adjust itself. Most of the shots came out right or close. I would suggest you follow Bert's advice of a minimum of 1/400 on the shutter, but consider going faster, and let the camera choose ISO. The D800 has so much dynamic range and low noise, you should be able to shoot ISO 1250 without any appreciable noise in the shots (if needed). In this case, a tripod would probably slow you down. A monopod may be helpful though. I have come to LOVE my monopod.