64 GB works great in the MT4GS. In fact, if you're getting the SanDisk Mobile Ultra card (are there alternatives yet?), that's my microSD of choice. Not only do you get the space, but it is the most practically responsive of the several brand / class combinations I've tried (good sequental and random access speed, I believe).
The one bit of bad news is that you'll need to reformat the card to FAT32, so the filesystem will be less space-efficient for small files, but that is a pretty small complaint.
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