Congratulations on your new D5100. As long as you will be shooting Sports outdoors in daylight conditions, the Nikkor 55-300mm f/4.5-5.6G ED AF-S VR DX is the value leader and optically excellent. More so given the price.
I would recommend reading the FAQ's pinned at the top of the Tripod forum before choosing one.
I would recommend getting an SB-700 or a used or refurbished SB-600. While the SB-400 is much cheaper, it is only slightly more powerful than the built-in Speedlight. Good Luck and Enjoy your Nikons!