My lay midwives and my gynecologist (who is also a CNM) both recommended an extra 300-500 calories since I'm still nursing my 27 months old. That's not a whole lot, a glass of OJ and a cup of yogurt would get you that much. I think every one is different, though. I like to eat a little more protein when I'm pregnant, so I just try to add a serving of cottage cheese or be sure to make some kind of beans and rice dish. If you are into the whole Bradley birth philosophy, they really push eating eggs, I think 2 a day.
Every pregnant body is different, though, so as far as how many total calories, I would just take your noraml diet and add an extra snack or two. Of course, if you feel fine and your nursing baby is happy and your baby is growing well, maybe you don't need to change anything!!