How big of a stone?
My advice: don't limit yourself to the D color (flawless). You really can't tell the difference between a D and an F - even the pros have a hard time telling the difference. I've heard/read that different graders can grade the same stone with D, E, or F just because they are all colorless, and it can be near impossible to tell the difference between them with the human eye. Not only will you pay a ton more for a diamond of D grade, but you will really limit your possible selection. Unless you have to have a D diamond simply to say "my fiancee's ring has a perfectly colorless diamond," then I would consider anything from D-F.
I don't know if you are planning on yellow gold, white gold, or platinum (but looking at the specs you are looking for, I suspect you are going platinum), but if you are planning yellow gold, then you really don't need to spend the extra money for a D stone, since the stone will pick up the yellow color from the ring anyways.