Yea, before i read this post, seeing your first one, that looked like the stock firmware. That certainly can't handle 256gb, not iven sure about 128gb. I also think it has the limit almost all firmwares had until the very latest ones. Limiting the number of tracks to 5800.
I bought a 256GB card to replace my 128GB, only to find i had a load of hidden files taking up space on my old card. Decided to use the larger card in my tablet to benefit from it's space. Still have around 25gb free in my X3.
Interestingly, the info menu says i have 0 songs. I guess i must have broken the number it can display! But when I update the media library, that shows the actual number i have.
I tend to only have 320kbps MP3s. As I don't really see the point of better when a great deal of recordings already out there are not done at a much higher bitrate anyway. Even 16bit flacs take up over double the storage and sound ever ever so slightly better, and you can only really tell if you use the right listening equipment in a quiet area. And from my experience with the one and only 24bit recording I got was that this particular one seemed a bit pointless. The recording itself wasn't particularly outstanding. It barely benefited by being in this format. For those always wanting the absolute best and don't care about storage space, I can understand. But I often listen to music that is 20 - 50 years old, and the CD quality flac and 320kbps Mp3 has next to no difference sound quality wise.
Anyway, you can get the latest firmware version here:
http://www.fiio.me/forum.php?mod=viewthread&tid=40779
Or Mr Trev and others did a custom firmwhere which tidied up the main menu which i think is a bit of an odd layout. But this firmware is a huge improvement over the first. The one area I don't like about the new one is the volume steps. Is now 120. So now, 2 clicks up or down will be the same as 1 for you now. Takes longer to get to different volume levels. I personally think there should be an option in the menu to skip 2 steps at a time or have the maximum volume at 60 so it is the same as the old one. But everything else about the firmware is an improvement. Even sound quality wise from the headphone output.
Regarding the SD card, I think Integral are one of the best brands out there value wise. I'm not a fan of SanDisk. They charge a great deal for their cards and you can usually get the same performance from Integral at a lower cost. Plus Integral were the first brand out there to create a 512gb micro SD. I remember a time when i got some Sandisk cards that happened to be in a bad batch. I just don't think Sandisk are as good as other brands for the price they charge.
With the X3, I think you may as well get a very basic card anyway. The read and write speed makes no difference to the player's performance and if you won't be putting things on it often, you may as well save and go for a budget high capacity card.