Toki, congratulations!!
I LOVED breastfeeding my babies... it was such a beautiful bonding time. Something that no one else could share.
I did find something about South Beach online... "Dr. Agatston states that the South Beach Diet is acceptable for breast-feeding moms as long as they have their doctor's approval. However, it is very important that they follow Phase 2 of the diet rather than Phase 1."
Certainly if you're currently eating chips and chocolate all day, a lifestyle change is a good idea where food is concerned.
I would definitely talk to my doctor about it, though.
We're here for support anytime... best of luck to you, and enjoy your baby time!!