Im a big believer in the benefits of upgrading power cords. But the results vary significantly depending on what equipment you have, the quality of your mains supply and the type of power cord you buy.
Ive had Nordost vishnu power cords before and was very impressed with their results.
Also, a very small company here in Dublin called Dalkey audio, makes and sell's high quality cables on that auction site. The cheapest being made of silver plated copper wire and the most expensive made of pure silver and includes a silver plated mk plug and wattgate 350 IEC.
These cables are well made and sound as good if not better than the Nordost cables.
The best cables ive found are Ps audio's premier sc. I have 2 of their 2meter cables which retailed for $1800 when released in 2006.
They are now discontinued and i picked them up for $280 each. The downside is they have US plugs on them so you either need to install a US socket in your house or buy a power filter with US sockets like i did.
All the power cables ive tried improve dynamics, detail and realism and bass quailty/quantity. The better the cable the more you get.
The Ps audio cables were the best and gave huge power to the bass and superb micro/macro dynamic ability and also actually made the system sound louder than before.
Any power cord will only really shine when you fit an audio grade fuse like a hifi tuning silver fuse or an isoclean one.