I'd agree, with three batteries it's hard to fill up a 32GB in a day and you ought to be downloading (then format in camera) at every opportunity.64 mb - plenty
I have two 32GB but have only needed the second one once or twice when main one has been forgotten in card reader at home.
While Sandisk are good they are also the most faked, I have generally gone for Toshiba cards and had no problem.