Note: The D40 is not equipped with a built-in focus motor (like higher level Nikon cameras.) So you will *not* be able to find a very affordable lens ... if you want autofocus.

So, you *must* use an AF-S lens. (Those lenses have a built-in motor that provides autofocus with any Nikon camera.)

Some Sigma lenses also have such a motor, called HSM, but the 18-200mm lens does NOT. So, you would not have autofocus!

The Tamron 18-270mm lens DOES have a motor too, so it would provide autofocus with your camera. ($629) Not super fast autofocus perhaps. This lens also has an Image Stabilizer.

See the test report at

http://www.dpreview.com/lensreviews/tamron_18-270_3p5-6p3_vc_n15/page4.asp

The Nikon 18-200mm is very good for a lens of this type. $700. (It's very difficult to make a wide to telephoto lens at an affordable price; hence, none of them are superb).

See

www.dpreview.com/lensreviews/nikon_18-200_3p5-5p6_vr_afs_n15/page4.asp

Usually, Sigma is specific as to whether a lens provides autofocus with the D40, D40x and D60.

For this lens, they do not mention that at all on their Web site.

**Many Sigma lenses in Nikon mount do not include the necessary focus motor for Autofocus with those Nikon cameras.**

But the 18-200mm DC II lens does, apparently.

...Sigma has announced that it is to make its unstabilized 18-200mm F3.5-6.3 DC lens compatible with Nikon's D40 and D40x SLRs which lack an in-body focus motor. ...

If you do not often shoot in low light, I don't think you will miss the stabilizer too much. Use a higher ISO level to make sure you have a shutter speed of at least 1/350sec. when using the long telephoto focal lengths.

Cheers! Peter Burian,
Contributing Editor, Photo Life and Here's How magazine