The best way to see orientation effects is with a 2D XRD experiment. The diffraction rings that are observed for a randomly oriented powder will no longer be equally as intense about the diffraction ring, Instead you will have areas of brighter and weaker intensity based on how the molecules are oriented within the sample and with respect to the experimental sample orientation. See Attached diffraction patterns of a polymer. As for polycrystalline vs. powdered samples, they are typically the same. Powdered sampels are polycrystalline if they do indeed diffract. If you mean that the powdered samples are dominated by large oriented crystallites, then your diffraction ring will not be a solid intensity but will instead be a spotty mess because of the each large crystallite will be diffracting as a single crystal.

In case of oriented samples there is a significant reduction of  most of diffraction peaks intensities. In some cases only one peak can be observed on the diffraction pattern.