The Sigma 17-70 is not a true macro lens, it just has close focus abilities. I'd look into one of Sigma's macro lenses. depending on what you shoot, 50mm or the 105mm. I use the Canon 100mm myself.
I do not have any Macro lens myself. But I came across with people talked about the Sigma Telephoto 105mm f/2.8 EX DG Macro Autofocus Lens is pretty good. And the price is around $350 new. And of course, the Canon 100mm also got great reviews as well but cost a little over 500 bucks.