I'm surprised the SD slot can read 64GB or greater, Nintendo's official statement is 32GB max with updated firmware. For that matter, if it can read SDXC natively that's a miracle, since some devices need those cards formatted FAT to work.
I bought a Samsung EVO 32GB SD card yesterday and copied all the files form the 16GB card that came with the system(bought a pre-modded console from another member). It gives me some extra room for Gamecube ISOs. Although a 64GB card, if it does work, would let me expand that further and give me more than enough space for Gamecube games. But for Wii games I need a flash drive or SD card/USB adapter, but I suspect even 64GB won't stretch too far given the ISO sizes. What's better in that regard, SD/USB adapter, flash drive or external USB HDD?