Be sure you buy Series 10 cards. These are the ones marked with a "C" containing the numeral "10" and are the highest speed currently available. The cards are rated numerically 2,4,6, and 10. I don't remember what the numbers signify, and they may not all be available any more, but the 10 series is (or used to be) the fastest. Older, lower image size cameras could get adequate speed with series 6, but your D600 needs the highest performance.