How many watts RMS do I actually need?

RMS (continuous) power matters far more than the headline 'peak' or 'max' watts printed on the box, which is marketing. Match the amplifier's RMS rating at the impedance your speakers or subs present (commonly 2 or 4 ohms) to your speakers' RMS handling. A sub rated for 600W RMS pairs well with an amp delivering roughly that at the matching impedance; overbuying peak-watt numbers does not help.