SDHC (SDv2) only goes up 32GB. Most of the current SDXC (up tp 64GB) is based on SDv3, which can be used on a SDHC slot if the firmware / OS has the driver. But given that (1) there is no microSDXC in the market now (only normal SDXC), (2) most normal SDXC with SDv3 is stuck on 64GB and (3) SDv4 (64GB and up, and not compatible to SDHC slot) has been announced early this year, there is a very good chance when manufacturers start making microSDXC, they will make it to SDv4 spec, which means we will see 64GB+ microSDXC that can not be used on older device, including X3. In layman's term, there is very little chance that we will be able to find a microSDXC that will work on X3 and most of current DAP with SD slot. 32GB is likely the best of what we can get. If you look around, even Sandisk (which is one of the largest SD card manufacturer around) doesn't have a microSDXC compatible slot on their latest Clip Zip.