20w-22w RMS per channel. The sub will need it's own amp. If your not going to add an amp for the interior speakers then don't get high power speakers. Go with entry level models rated for 25w-30w RMS per speaker. Be aware that most interior speakers are rated using Max/Peak ratings for the PAIR so when shopping find out the RMS rating and divide by 2. A good match for the stock radio would be speakers that are rated at 50w RMS per pair.