What will determine the gauge of wire you need to run to the Opus is determined by the distance from the battery. if you were putting the computer and powersupply in the back of the car, you would probably want to use 8ga. wire feeding the Opus from the battery, because using a smaller wire that long would result in a voltage drop. You would see maybe 11 volts in the trunk due to the resistance of that 12-10 gauge run to the trunk. BUT you seem to be locating the computer and powersupply in the dashboard. You've got maybe, what, 6 feet or less to the battery? i don't think 8 Gauge wire is really necessary. Since the distance is short, use 10 Gauge wire. Make sure you also fuse it within 8 inches of the battery terminal also. The main thing you will have to pay attention to in order to prevent noise will be your grounding.
1. Make sure your ground wires are the same gauge as your power wires.
2. Make them as short as you possibly can.
3. Ground to an area free of rust, paint, grime. CLEAN METAL CONTACT.
4. Best ground connection is made Like this: