The salesman was in fact technically correct. Gold is the best conductor and it doesn't corrode. The cheaper cables have nickle/silver-plated plugs - while the nickle/silver alloy won't easily corrode, it is also not a very good conductor. Audibly gold connectors will make some subtle differrences espeacialy at the higher frequencies.
However, the more important factor IS the quality and guage of the copper wire. The more pure (99%) and heavier the gauge, the less resistance and therefore the better the transfer of ALL frequencies. This is quite audible. The highs will be crisp and transparent and the low end will sound full and natural.
Using lower grade cables is like trying to squeeze the output of a fire hydrant into a garden hose.
The same is also true for speaker cables - regardless of the size of the speakers or the power of the amplifier. Use 10 guage Monster type cable w/ 99% pure copper wire. Even in a 20 ft run the voltage drop and frequency loss will be negligeble. Your ears will be smiling!