My alternator dying?

I've noticed the last few days that wen im driving my dart with my headlights on an wipers going, at idle my gauge reads closer to D an wen I have my foot In The gas, it innediatly goes to the C side. I Thot it was suppose to stay in the middle. Not sure if it does it without lights off. Also I notice when I hottie gas, the lights get brighter. Do I need a new voltage regulator or alternator? Is there something else wrong here? Thanks