I got a component cable from my local game store about a year ago, it is Wii, xbox 360 and PS3 compatible and it cost about as much as the official Wii HDMI cable. The quality of the picture I get from it is excellent, just as good as when I plug my PS3 or Xbox 360 in via HDMI.
silver and copper (Which is what those cable connectors are made of (copper), they just look gold ;-) ) are better conductors...but then again this has nothing that much to do with how the signal is output. The problem lays in what is used inside the cable. Lower grade copper wiring (which is often used in cheaper products because cheap production costs) will reduce the quality of picture being output....I think.