Personally, I believe that a good healthy and balanced diet if perfect when pregnant - totally agree with Hanadr regarding nutrients and empty calories. If you can keep youself healthy, then the baby will take what it needs from you, and in all likelyhood you will maintain or reduce your weight during your pregnancy. This is certainly what my OB recommended to my when PG, and certainly none of this 'eating for 2' mullarkey.
You do need to be aware of your calorie intake during breast feeding, however if you consume an additional 3000 per day you will wind up the size of a small village (Sorry Hanadr). 400-500 extra per day should not only maintain your milk supply, but should also set you on the right road for losing additional weight (within reason) post natally.