I have used the 45x Kingston 8 gig cards and the Hoodman RAW 280x 4 gig cards (and others) in the Leaf 65. From what I understand, the cards are all caapable of writing faster than the back.
However I think I did see more of a delay in the rendering of the image on the display faster with the faster cards. Totally anecdotal on my part but my observation.
But there is a big difference in download times.
I do believe that Leaf recommends using the fastest cards available.
