with my first, I bought nursing tops and don't suggest them - they are ugly and don't work very well. My favorite nursing bras for starting out are from A Pea in the Pod. They are soft - you can sleep in them, but they are moderately supportive. I use them for sleeping now, and after baby is born, and will be bringing them to the hospital with me. They are very stretchy and will allow for changing boobies, and also allow you to leave the house!
You can get more supportive nursing bras once your milk is in and you can measure yourself how the PP described in her post.
I also have heard of a company called undercover mama. They sell tops that basically hook up to your nursing bra, and turn it into a nursing tank. I am going to try that, because I would rather spend $$ on great bras, and then just use that tank when needed. They are $28, which is cheaper than the good nursing tanks.
I tried the target brand nursing tanks, and they sucked! I think they are probably good for smaller chested women, but I am large-chested and petite, and they were horrible for me. I never wore them and actually just threw them out.