Eh sigma and tamron are kind of shoddy quality, at least compared to Canon and especially Nikkor lenses. My father has a Sigma ultra wide angle lens and I was really disapointed with its performance. If you want a great zoom (well up to 200mm) I'd get a Nikkor 18-200 w/ VR. Its probably the single most versatile lense made by any company. It can focus throughout the whole zoom range at around 1.5 feet or so in front of the camera, which means you can get some really good macro type shots with it. Not to mention its easy to shoot at low speeds 1/10 ->1/15 with the VR and get ~50% of your shots in nice focus.
So as long as yoru camera will accept Nikkor DX lenses its probably the best single lens you can buy.
Oh, and two great, reputable places to look for lens are bhphotovideo.com and adorama.com. Both are the websites for two amazing stores in NYC, and are considered some of the best stores to buy stuff from by professionals. I've seen B&H have some really good deals from time to time too, but in general if you find a lens for a consierably cheaper price than most other places, its gonna come back to bite you if you buy it there, and also on that note, its somewhat rare for lenses to go on sale.