I did the exact same thing as your mom and trust me, it REALLY helped out a lot!! Here's what I did: I bought sizes 2,3,4,5,6 and then I would start over again. Depending on how much longer you have to go you'll end up with a few boxes of each size and that way you won't have to buy many once the baby's here. Also, I think we had about 7-8 big boxes of wipes because you'll go through TONS of wipes too.
I wouldn't buy formula because you might end up with a certain kind that your baby won't take.
You'll probably get lots of NB and size 1 diapers and your baby shower, we did and we never had to buy any of those sizes, and we actually had so many boxes given to us that we were able to go and exchange those for bigger sizes.
Edit:: You could start buying baby wash, shampoo, lotions, desitin, baby tylenol, mylicon, spoons & sippy cups (you'll be surprised at how fast they grow into these!) Also burp cloths, we could NEVER have enough burp cloths!