The D800 manual (now available for download) mentions 128GB CF cards and 64GB SD cards. That doesn't mean that it won't support larger cards in future, just that those are the largest that are currently tested and "approved".
I'm not sure what you mean by the "MD accessory"? The D800 supports dual cards (one CF and one SD) natively.
Lexar LECF1000X64G 64 GB PRO COMPACT FLASH CARD (1000x)
The manual does not say that it will support either a 64GB CF card, or a 1000x speed card. So, there I go trying to get the best and I may have bought something that won't work. But, I will try it and see.
And, I bought a
Lexar LESD133X128G 128GB Professional 133x SDXC Card
The manual doesn't say that it will support a 128GB SD card.
Of course the manual doesn't say that these won't work, either. So, we will see when it arrives.
The D800 Users Manual lists Sandisk Extreme Pro 64GB and 128GB CF cards as Tested and Approved. The Lexar 1000X CF cards were not tested, though I suspect that they will work without issues. The D800 approved SD cards are listed up to 64GB. The Lexar 128GB SD card is SDXC compliant so I suspect it will work as well. Good Luck and Enjoy your Nikons!