Have you asked her opinion yet? Maybe she doesn't want the biggest diamond you can afford. Maybe she doesn't even want a diamond. It all depends on the person, and to a lot of women, size *doesn't* matter. Do you know what kind of metal she likes? (yellow or white gold, or platinum?) Would she care if it was a conflict-diamond or an ethical one? etc.
I think these are more important questions than the technical specifications, but that's just me. You know her better than any of us do.
