Right now I have a D40x with the kit 18-55 lens. I shoot 90% Motocross photography, 5% skateboard and bmx and another 5% on random stuff like macro and night shots, family stuff, etc. Being a teenager, on a limited budget, I'm looking in the $400-$700 price range. Right now, I have the Sigma 70-200 f/2.8 HSO in mind, but they call it a macro lens. Would this lens still be good for shooting sports? Any suggestions on any other lens that would suit my needs, within my price range?