Where are you running your wire? How long are your runs?It matters a great deal if you are talking about power wires. It matters less with speaker wire.
There are quite few benefits of upgrading your wire even if you don't feel the immediate need for it. Future upgrades to your power supply are always something to consider preparing for. Larger wire means less power loss due to heat from smaller gauge runs as well. I wouldn't run 0ga if you never plan on anything bigger than 500w but say you might want 1000+w someday. Tearing out the 8ga could be a b1tch.
If you are going to have a big system someday, like 1k+wrms just get the 0awg now and a distroblock. Saves you money on the wire rather than buying it 2 times or more, and time tearing up your car installing it.
An average length of 4 gauge can carry a max of 120 amps of current (approximately 1200 rms with a class d amp). While that is ok its not very efficient and should be upgraded to larger wire.
