In all honesty I read a lot about card speeds and suitability for various requirements, but have come to the conclusion that unless one is a "machine gun" shooter card speed makes little difference on the D800. After all, the rather slow fps rate compared to either my old D700 or my current D7000 makes fast cards not quite as much an issue really.
If I was starting again I would probably have saved some money and have bought x400 cards. If I need the faster fps I take my D7000 equipped as I do with x600 SD cards.
Although perhaps not relevant to this discussion, I held off buying the D800 for a long time because of the slower fps in hope that a D400 would appear. I have learned that in fact for 90% of my shooting the D800 is more than adequate.