What bra size would 850cc's give me and would it look good?

The only thing certain is that if you go from 425 to 850, your breasts will be VERY LARGE.  This has been associated with increased breast sagging, capsular conjecture, and the feeling to the nipples.  In addition, the implants may ripple or become more visible.  Since there is no true standard when it comes to bra sizes, it's most likely that if you go from one bra store to another, you will be called a different cup size.When choosing implants, it's important to pay attention to the whole picture and seek the advice of a good surgeon, and far less important to fixate on a particular size of implant.  If you were buying a house, would you want a great looking 2500 square foot house or a run-down, shabby-looking 5000 square foot one?Best wishes,RM

While there are certainly more complications associated with such large implants, if you are a patient with a larger breast width, looser skin, and little existing breast tissue, then a bigger implant might better accommodate the breast size you are hoping to obtain. Although a specific bra size is not determined by implant size alone,your cup size would certainly be larger than a D with 850cc implants. Be mindful that implant extrusion and possible sagging of the breasts may occur long-term if you choose to have larger implants. I suggest you schedule an appointment an American Society of Plastic Surgery certified physician for top results. 

Thank you for the question.  despite your description of your situation, online consultants will not be able to provide you with specific enough advice to be truly helpful. There is no specific correlation between size of breast implant utilized and cup size achieved. Furthermore, what “looks good” to one person is subjective.  In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
Generally speaking, the best online advice I can give to ladies who are considering revisionary 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're looking for. Ask to see lots of examples of his/her work.  
2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or 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 "C or D cup" or "proportionate" etc means different things to different people and therefore prove unhelpful.
 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.  The use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
I hope this, and the attached link ( dedicated to larger breast augmentation outcomes), helps.