The 18-70mm which comes with the D70s seems pretty good.  Do you need the
longer telephoto range up to 200mm as a longer range can compromise quality
for a given budget?

I recently bought a Nikon 28-200mm for my D50, and am quite pleased. Not
quite as wide as the Sigma, but from what I read, much sharper,
especially at the long end. I just leave it on the camera as my everyday
lens now. It cost around $300.

Bob B.

Don't think that Sigma will be better than your current lens, so you're only
buying a rather mediocre short tele if you go that way.
A better choice (from all I hear) would be the 18-200 Nikkor (which of
course is more expensive, but well worth the extra money).

In your case you'd be better off with a 70-300 IF-ED Nikkor for now.

The Nikon 18-200mm AF-S VR DX Nikkor is almost certainly worth the
extra money. I have yet to read a bad review of this lens and it is my
own favorite travel lens. It stays on my camera almost all the time.

I doubt very much that the Sigma has anywhere the capability of the
Nikkor. Heck, you can even use the Nikkor for light macro duty. And the
VR works -- it really works. A guy can shoot at 1/8 of second with the
lens at 200mm and still get a sharp picture one time out of three. You
can do it every time at 1/30.