Does Gigabit require CAT6 wiring and base-100 require CAT5?
Also, if you were to run new wire, I assume you would want CAT6. Is CAT6 wire totally backward compatible to networks and equipment that would normally run CAT5?
I have a house in which the builder ran CAT5 in the main and upper levels. The basement has my home theater and main stereo system. I am thinking about adding wiring to the basement (assuming it is not too much of a hassle/expense) to connect my NAS's (Zyxel NSA320) WAV audio files to both systems.
Your CAT5 will probably work just fine. If you were running new cable you would pick CAT5E. You could use CAT6 (yes, it's compatible), just more expensive and unnecessary. Note that the cable ends or termination points also need to meet the spec as well as the actual build quality at the terminations points.