I would recommend doing a huge amount of research and asking a lot of doctors before you consider having it done. In particular, make sure you know the LONG-TERM success rates. The thing is, fat is living tissue, so it's not as easy as just sucking it out of one place and moving it to another. They have to somehow give it a blood supply, etc, to keep it alive, which makes for a really major surgery with lots of chances for something to go wrong, especially infection. It's more of a transplant than just a transfer.
And if you're working on losing weight, hold off on the breast enhancement until you're at goal. A lot of times, losing weight elsewhere makes your breasts look bigger. If you get the surgery and then lose 20 pounds, you might end up looking out-of-proportion.