have a look at this FAQ.
http://www.mp3car.com/vbulletin/advf...iew&advfaqid=5
It has a good table that shows you what gauges to use where depending on current draw and length. Generally you up those gauges if the wire is being hooked to something that is sensitive to noise (like an amp). Depending on where you're putting your components, that will determine the gauge you will need.