DSLR Camera Lens Help? (NIKON)?

I'm using Nikon D3100 with Lens 18-55mm VR. I want to buy a new lens I was thinking to go for 70-300mm VR.
Would it be a good decision or I should go for some other lens?
And what are sigma lenses?
Are they worth buying? Do they effect the image quality, focus, zoom etc as compared to the original lenses?

The main problem with buying a lens like this is every time you're out and about and you want more zoom you have to change lenses. Every time you change you're opening your camera to dust that can find its way to your sensor… Not good.

I'd recommend more of an all around lens that you rarely have to change, the 18-200 comes to mind.

You buy lenses as you need them, not on a whim.

I carry 18-55mm and a Sigma 55-200mm for my DSLR - while it may be inferior in bench tests to the manufacturer's own, it's hard to see the difference in the real world, just try before you buy.

I frankly can't understand the modern fixation with a single, long zoom - you are compromising image quality here, as well as carrying a big, heavy lens that makes you reluctant to take it with you. I cut my teeth on 35mm and usually carried 28mm wideangle, 50mm standard and 135mm telephoto - a kit that could handle most situations.

Also make sure that any lens you buy is motorised (Nikon AF-S/Sigma HSM) because a non-motorised lens won't autofocus on a D3100.

I have never felt Sigma had as good of quality as Nikon. Sure, there are exceptions, but anytime you go to the aftermarket, you are usually going with less quality.

Now, it may be a situation of measurable vs.noticeable, so you will have to judge that for yourself.

Here is a webpage showing the various focal lengths, and what you can expect from each lens:

http://www.althephoto.com/concepts/lenses.php

You can see that the 70-300 will have a significant improvement on your ability to bring in distant subjects.