Like richx likes to say, 18-200vr isn't that great. Just useful for convenience. But for me, I can live with its compromises and absolutely loving it.

APO for sigma is like ED for Nikon. Low dispersion glass. Having this = better optical quality but more expensive of course.

Edit:
"In order to attain the highest quality images, the APO lens has been made using special low-dispersion (SLD) glass and is designed to minimize color aberration.
SIGMA's APO zoom lenses minimize color aberration. As the refractive index of glass depends on the wavelength of light, color aberration occurs when different colors form images at different points. This problem often occurs with telephoto lenses, but the Special Low-Dispersion (SLD) glass and Extraordinary Low Dispersion (ELD) used in SIGMA's APO lenses helps to compensate for color aberration, thereby allowing them to produce of sharp images."

This post has been edited by ferox: Apr 26 2006, 10:11 AM

I would keep the 18-70mm.

Someone on photomalaysia selling for RM480
Sigma 70-300 APO Macro Super 2 - Nikon Mount
Click here

It will work on your dslr.