Should I get 400cc or 450cc? Is there a big difference?

Don't get caught up in fretting over actual number of cc's of your implants. There are so many other factors involved, like your height, weight, body type, size of your breasts and Breast diameter. I ask patients to show me photos of breasts they desire, a good starting point. I then use various sized implant sizers with bras and a T shirt to allow the patient to see what she likes. This narrows it down further, and lots of conversation with the patient gives me an excellent idea regarding implant size. Seek out a board certified plastic surgeon to help you decide.

You should try sizers on to determine which is the best for your personal perception of size.  If you want to be a D, and you choose the smaller size, you will most likely be disappointed later.

Thank you for the question. Depending on a specific woman's anatomy and goals ( in regards to outcome achieved with breast augmentation surgery), 50 cc may make a “big difference”.  For example, for a petite patient striving to achieve a relatively “modest” outcome, 50 cc of a larger volume implant may be very noticeable. Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery (regarding breast implant type/size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "small D cup" or  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery ( after the use of temporary intraoperative sizers).
I hope this, and the attached link (dedicated to breast augmentation surgery concerns), helps.  Best wishes.