The Sigma is a lot of lens for the price, but it is pretty soft above 200mm, and without VR, you'll be married to a tripod for most usage.
As for macro, it is only 1:2 at 300mm. Did you know your 18-55 is capable of 1:3 at 55mm, and it is sharper with more depth of field.
As for telephoto, the Nikon 55-200 is sharper, and the VR means you'll actually be able to use it hand held. It is a very good budget lens, especially if you go with the VR II version.
Unfortunately I have no experience with the tamron (never really liked them, but hear the newer ones are much better).