Bad alternator?

I just got back from a little drive with my 64 Dart tonight. I haven't had it too long and took it for a drive on the highway.

It ran great while on the highway, but when I got back in town it wanted to die at each intersection. The oil light would come on and the voltage gauge would drop down towards D (discharging?). I had to put it in neutral and keep it reeved up at each intersection so it wouldn't die.

The other night a friend told me it was acting like it was running on the battery as the lights would get brighter when you gave it gas, but it didn't act this bad.

It's probably an easy fix and I'd give my friend a call but it's getting late so I thought I would see what you all thought.

If I do need to get a new alternator should I upgrade to a bigger unit?

Thanks for the help!