How significant is a power cord upgrade?

After going through a bunch of pretty good equipment, I have yet to hear cables that have the sonic impact of an upgrade to my source, preamp, amp or speakers. I have had good luck with the previously mentioned Belden cord, the Asylum cord (Belden or Volex with better ends), the VH Audio cords, and some home brew cords using the VH Audio wire, and special shielding/vibration damping material with high quality ends. Before I would venture into the $500+ PC territory, I would try to have everything else dialed in first, including speaker cables and IC's. To answer your specific question, I would start at the lower end and work upward. You can't really hurt anything going with the Belden/Volex or Asylum cords, and those should be an upgrade to your stock cords. Another tip is to clean and polish the ends of whatever cords you use. Deoxit and ProGold, or Brasso should work well. I would also suggest borrowing a higher priced cord from a buddy just to see if you notice a difference, before spending big bucks on one.

"How significant is a power cord upgrade?"

In my experience, less than that you'll get by addressing the room acoustics.

Science or snake oil. The jury remains out on esoteric power cables. Ever wonder why the likes of manufacturers such as Krell, Pass Labs, Levinson and others, do not supply esoteric power cords? Makes one wonder in a way. After all what those products cost and few dollars more is of zero consequence to provide a power cable,other than what is usually supplied. Although I use aftermarket power cords myself for amp,preamp and CDP. For me where I live is more of self survival. Seeing as Florida Power & Light continue to struggle with their service after the hurricanes of 2004, and 2005. Power cords I have used is DH Labs, Audioquest,PS Audio and Shunyata. And at times have returned to the cable that came with the unit.

I think to get a good idea of what a power cable is faced with, we need to visit the Nelson Pass white paper on Power Supplies. link below.

http://www.passlabs.com/downloads/articles/powersupply.pdf

In this paper we see the end delivery and what the power cable is actually faced with in certain amplifiers and the components used in those products, such as transformers, capacitors,etc. Interesting reading at the very least.