Check out the 70-200 f4 IS, great for outdoors, kids sports, etc., that'll be in 1k range, else get the 70-200 2.8 w/o IS which would also help you indoors with low light. Better to have the constant 2.8 or 4.0 aperture throughout the zoom. Or if you want to stay in the $500 range, just get the 70-200 f4 w/o the IS. "L" quality is always a good thing

Invest is good glass, you should only have to buy that once.