The stiffness of a wheel is down to the strength of the rim, combined with how much tension you put into the spokes.
Funny this should come up, I was reading something about it a few days ago; according to Sheldon: "Some believe that a wheel built with tighter spokes is stiffer. It is not. Wheel stiffness does not vary significantly with spoke tension unless a spoke becomes totally slack."
http://www.sheldonbrown.com/rinard/wheel/index.htm