400-450 Cc HP Silicones Undermuscle for Petite Woman. Will Be Enough for the Desired Look? What Bra Size Am I Gonna Be? (photo)

You have a slender body with A cup breasts. A 450 HP implant will get you in the D cup range and should get the look you want.  I agree with prior posts to try the implants on in the office to get a rough idea of the result.  A periareolar incision is recommended due to your poorly defined inframammary fold.  Best wishes

In regards to incision choice, I think a well placed crease incision can be camouflaged very nicely and carry less risk compared to the peri-areolar scar in terms of sensation change.  In order to make an accurate size recommendation, I would need to assess your chest wall and breast mound measurements and characteristics.  Unfortunately, there is not a general rule of thumb or objective criteria to implant selection.

Your plastic surgeon will perform several measurements of your chest wall and breast anatomy and determine a range of implants that both fit your chest wall and reach your desired goals.
The next step is to try on this range of implants in the office with your doctor.   The key to this success is showing your surgeon the body proportion you desire with a bra sizer and allowing your surgeon to guide you to the right implant.   It will be much easier to communicate in implant cc's than cup size when determining the appropriate implant for you.
I wish you a safe recovery and fantastic result.
Dr. Gill

From your desired photos it appears that you want a very well defined rounded look.  If you use a HP 400 to 450 cc this will give you the effect you want.  You just need to be sure that your actual base diameter is not smaller than the width of your desired implant.  Also Vectra 3D imaging can be very useful as well.  

In my opinion, the best way for a patient to choose the right implant size is to try out different implant sizers with a tight tank top on to see which size she likes the most. Base on your desired implant size, your surgeon will choose the appropriate implant profile that would match your breast base diameter. Given that you have a narrow breast base width and desire for a fuller, most likely a high profile or extra high profile implants would be right for you. Good luck on your quest.

Since there are no fixed definitions of cup size, I don't see much value in a prediction. Furthermore, a C cup on a 34 is smaller than a C on a 36, larger than one on a 32, and so forth. On a 30 or 32 band, you might expect a 3 cup size (possibly more) increase. Try on implants, see if the size works. Your surgeon will likely try to limit your choices such that the implant is not wider than the breast. 

Best wishes.