I'm all for saving pennies, especially if photography is just a hobby, but since you have already invested in a nice camera body, and will possibly spend a significant amount of time in the near future to learn up photography, why not go for a really nice lens. Consider the 70-200 f4 L USM, can be gotten for ard $1K used off the B&S. Yes, $700 difference is a lot of money, but in this case I think its worth spending. You can still re-sell it for little loss later if you wish.
Btw, I used to have the Sigma 70-300 APO. A nice little lens, and I really liked it. But the image quality really pales in comparison with the the 'L' zooms.
Btw, I used to have the Sigma 70-300 APO. A nice little lens, and I really liked it. But the image quality really pales in comparison with the the 'L' zooms.