put the new amp in and see what happens first, the memphis digital line are extremely efficent and really dont pull that much power. i have a 70 amp alternator in my car and i have a memphis 1000d and a kicker impulse 4ch and the lights dont dim at all. there is only a need for an upgraded alternator when you have multiple amp s with high wattage, this is not the case with your setup so it should be more than fine the way it sits.
