Here's another technique for infinite good karma. If you head to Megaton, you can find the church for the Children of the Atom, religious people who worship the undetonated nuclear bomb which Megaton is built around. There's a priest who will be near the bomb or in their building. Talk to him and he'll ask for donations. Give him donations, and you'll get a positive karma boost. Do this as much as you want for as much karma as you desire.