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Charging system problems

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I have a problem with my charging system in my 66 Nova. Well maybe it's not the charging system as much as it's the stereo system. I just installed a complete competition stereo in my car last Wednesday. It has 1 800 watt amp for each of the 3 JL Audio W6-15 15 inch woofers and another 400 watt amp for the mids and 200 watts for the highs. I count a total of 3000 watts. I know my charging system is WAY over worked, I can see the head lights dim dramatically when I drive, even at high way speeds, and I have been having problems with the amps cutting off after a short while because of low voltage. I don't know the formula to convert wattage to amperage so can anyone help? I thought about changing to a CS130 alternator off of an 80's car. Will this produce enough amperage? If not what alternator should I use? Are there any factory alternators that will work? Or will I have to go with an aftermarket alternator? I saw one in a magazine rated at 140 amps, is this enough ?
Thanks for the info ahead of time.