Ethernet Cables -- where do they matter most?

Question:

I am always looking to squeeze goodness out of my modest system.

Ethernet cables really didn't occur to me as an upgrade path.

Where do these cables matter most?

My set up is as follows:

1. modem -- > ethernet runs approx 100ft run to router
2. router --> ethernet approx 2ft to netgear switch -- connecting to pc, dvd, roku, av preamp
3. switch --> approximately 6ft Marantz AV7704 preamp

Do I have to replace the very long ethernet run as well  for it to effect sound quality of each piece of hdw that my switch is connected to??