I'm not a boob man, so anything B or natually larger works for me. I don't mind implants so long as it's not over done.
Like Mike Holmes says ... "Make it Right", if you plan to spend the money, do it wisely. There's nothing worse than seeing something that resembles a couple of pumkins that have been sliced and sewn onto a chest. On the same note, I've seen to fantastic jobs done that although are dead giveaways to being man-made but my fist thoughts upon seeing them involve a game of hide the sausage.
I've found that anything above going up 2 sizes is overkill ... but that's my opinion.
Dealmaker if it's done well and doesn't look like bolt-on apperatus, dealbreaker if it's the first, second and third things I notice about you.