Smoothing shadows and artifacts in HDRI render
 

I recently applied a tip from "Maya 5: Killer Tips" to create the attached HDRI rendering. With just a few actions I came up with this render. But I notice that I need to smooth out the shadows and get rid of slight artifacting showing in the image.

Can someone tell me what settings I should adjust to do this?
BTW, the only lighting comes from the HDR image placed in both the Color and Ambient channels, a light was added but the Intensity was turned down to 0.

http://www.nulightdesigns.com/maya/HDRI_Jan25.jpg

increase FG rays, and decrease color grain.
color grain in ambient.
go to the HDRI setting in the ambient channel>scroll down to color effect>adjust the color grain lower.