Cable differences -- power, interconnect, digital, and in what order?

A friend of mine has been a real connoisseur of interconnects and speaker cables; he hears very subtle differences between them, mixes and matches them with different results, and experiments with upgrades (and reselling!). 

One thing he's never done much with is power cords or power conditioners. 

This raised a question for me -- to what degree have people found differences in source and interconnect cables vs. power cables? Are they all important to an equal degree for you? Where is the greatest impact for you?

And if you do find power cables to be an important factor, where have you started making changes in your system? Wall to amp? With source? 

Thanks for any thoughts.