If you don't absolutely need the wide end of the 17-70, then another good choice in the price range would be the Tamron 28-75/2.8. For what this lens can produce, it's hard to believe that it is as cheap as it is.
If you do need the extra width of the 17-70, then most definitely get the Sigma. You won't be disappointed. If you do go this route, I would advise you to either keep the 50-200 or upgrade it to something like the DA55-300, if your finances allow it. Depending on what it is you want to shoot, the 55-300 can also be a good walkaround lens.
HTH,
Heather