you can put a 64GB CF card, price would be prohibative, but it would own,
the problem is that the higher the density in memory cards, the lower the speed, CF are great for this becuase of the integrated ATAPI controller,
another idea is that theres several SD to ATAPI convertors which allow the usage of several SD cards in a single array in stripping mode (meaning that a single partition is established across all the cards and the act as a single drive), using 4-6 high speed 8 or 16GB SDHC cards, interesting results can be obtained, but im not sure its price effective,