Got a quote from OP for the 70-300 Sigma for $270. It's a grey set of course with their shop warranty but it does not bother me coz their service is very good and I would only be concerned about warranty on my more expensive lenses ($1000+). The local warranty copy is about $320-330 as another fellow Cs'er mentions above.
Been looking for a nice zoom myself....started of thinking the Sigma 70-300 would be a great starter lens...nice range...nice lens...great value at that price and is very well reviewed everywhere...great feedback from many fellow CS'ers.
Then thought naaahhhh...might as well go in for the Nikon 70-300 VR coz the VR is an attractive feature at that length...a nice lens too and costs 2X as the Sigma option...but well worth it in my opinion.
Then again thought naaaaaaaaahhhhhhhhhhhh....bought a 17-50 2.8 and am totally totally 100% convinced that 2.8 is the way to go if you really want to stretch your lenses & cam...the massive jump in quality of my pics totally shocked me too....luv it! So now i am saving hard & fierce for a 70-200 2.8 (if i hit a jackpot along the way then it will be Nikon and if not maybe the soon to be launched Tamron 70-200 2.8)
Cheers mate..all the best...this is a expensive and a confusing hobby(sometimes frustrating too!) ...trial & error rules!! haha