Om my D800 with the EXPEED 3 processor it writes about 28meg a second using a ScanDisk Extreme Pro 95.
Like others have said a card with 45meg speed should be more than adequate. What I think is more critical is not to go cheap. I have always used ScanDisk cards and find them to be highly reliable. Most of the complaints you hear are from people saving a few dollars by buying off of Ebay where the seller and his products are unknown.