You can add a 64GB mSDXC, but it falls back to class10 mSDHC speed, & you need to reformat it to FAT32, as there's no SD3.0 support in SGSII.
Going with SDXC at least gives you the option of capacities beyond 64GB, but it's still very pricey, & won't be good value till approx. this time 2012.
There are better value 64GB SDHC's*, but again, FAT32 (so 4GB file-size limit), & poor R/W performance compared to the built-in storage.


* official spec says max is 32GB, but OEM's can bend the rules, & up it to 64GB if they want.