You can go from C cup to D with aproximately 220cc; but as your dsire is a "full D" size, I think you need 280-300cc.
Personally I don't think that letters/numbers are a good guide.
There are several useful methods to show you a very aproximate real look of your planed BA prior to surgery. In European Union / Spain, we use ALLERGAN "BIO-DYNAMIC" set; you can see in front of a mirror different implant sizes and shapes, guided with a software breast size analysis.
Another option is MENTOR "BODYLOGIC" measurement system.
Other software systems can also help to make your decision, like E-STETIX (CRISALIX) or VECTRA 3D (CANFIELD). These ones are virtual systems, but they are the perfect compolements to Bio-Dinamic or Body-Logic.
Good luck!