Heres what I think:

Speaker cables- 12 or 14 gage zip works fine, other designs affect the sound by attenuating the signal for certain frequency ranges creating the illusion of a brighter or warmer sounding cable when all they are really doing is turning down the bass or the treble. 12 or 14 gage zip produces the flatest frequency response in the audible range of frequencies (if you don't believe me hook up your $500 cables to a network analyzer and see for yourself).

Power Cables- All stereo components have DC power supplies in them so clean power is not really neccessary, in fact many electronic devices will work with a square wave AC power source.

Interconnects- Video cables can affect picture qulity do to the fact that RF noise is on the same magnitude as a video signal (10-100MHz) and video cables that are poorly shielded show. As for component interconnects, I use a $30 pair of interconnects for my CD player and they don't seem to do much more than provide a strong connection.

These are "my" opinions and reflect my somewhat limited knowledge of electronics.