I have Breast Aug booked for 23rd Dec. Will 480cc give me a DD cup?

If you would like a larger implant, then you should be indicating to the surgeon that you would like a larger implant.  There are no set rules for maximum size.  If you have tried on sizers and liked the 480, then that's one thing.  If the surgeon is choosing on your behalf, you will likely be unhappy.  Depending on your size now, 480 will probably not make you a DD.  I do not think tear drop implants look more natural.

There are many factors that go into choosing a size of an implant. These include your height and weight and pre-existing breast dimensions and size. A larger taller framed woman would require a larger implant to achieve a double D cup versus a smaller petite woman. There is also no standard as to what a cup size is. Some bra manufacturers consider a D cup quite small versus other manufacturers. It is reasonable to bring to your surgeon photos of others with an outcome that you are hoping to come close to. You can also ask your surgeon the differences between and anatomic textured teardrop shaped implant and round implants. There are pros and cons for both models.

Thank you for the question.   You will find that online consultants will not be able to provide you with specific advice in regards to selection of specific breast implant size/profile. Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, computer imaging and direct examination/communication in front of a full-length mirror) will be critical.  Given that your surgery is coming up soon, the best way to alleviate your anxiety is to spend additional time with your plastic surgeon, preferably prior to the day of surgery.
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery (regarding breast implant 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 "DD cup” 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.  Again, the use of computer imaging, may be very helpful during the communication process.
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).  Viewing the patient's chest wall in the upright and supine positions, with temporary sizes in place, help select  the best breast implant size/profile for the specific patient.
I hope this, and the attached link (dedicated to breast augmentation surgery concerns), helps. Best wishes.