[badpickev]

looking at glass you have a few choices for a standard dx zoom.

sigma 18-50 f2.8 - sharp and has hsm
tamron 17-50 f2.8 - a little sharper than the sigma but lacks hsm cheaper as well
tokina 16-50 f2.8 - a little pricey and is equal to 24-75, reports of soft at 2.8
nikon 17-55 f2.8 - has af-s and is the most expensive pros claim its worth the money

for a sports lens you want a fast lens and the bigma is not it. the bigma is f4-6.3. by 200mm its at 6.3. i have shot baseball games with this at 800 iso and after the sun goes down its done. the motion blur is really bad. so you are left with just the fast glass.

sigma 70-200 f2.8 - pretty sharp and costs about $900
tamron 70-200 f2.8 - still not in the us but pre release pricing is set to $700
nikon 80-200 f2.8 - cost is about the same as the sigma
nikon 70-200 f2.8 VR - has vr for hand holding and monopod use worth the $1600
sigma 120-300 f2.8 - has the longer range that may be needed, some reports of softness $2700

a cheap way for wildlife is a used 400mm f5.6 lens or you can go with a bigma.