Thanks to 3g, we have a link to a very frequently asked question: "What is the proper gauge for the power wires for my PC installation?"
Of course, this is dependent upon your installation. How much power does it draw and how far are you running the cables? The link above provides a handy reference for using the proper gauge of wiring.
Why is it important to have the proper wire size? I mean, after all, aren't we just talking about 12 volts here? We are. We're also talking about your car catching on fire from an electrical overload if you use wires that aren't sized for the load.
Say that with me again: F-I-R-E. In your C-A-R.
I've said it in previous postings on installing a PC in your car, but it bears repeating again; the supply of power to your PC is the most critical portion of your installation. Improper cabling, grounding, or power supply will at best cause your PC to crash unpredictably and at worst cause a fire.
You have been warned. Thanks 3g! Thread title changed to FAQ format and added to the FAQ to the FAQ's.