re: When will there be a 256gb memory card and will the Galaxy S5 be able to use it?
Going from a 64GB iPhone 5, with 1GB of managed free space (i.e. I kept it free for iOS operation), to a Galaxy S5 with a 128GB card was a big upgrade. A lot of people couldn't fathom wanting to carry around 6000+ songs on a phone and told me I was crazy when they looked at my iPhone's music collection. Now I have some breathing room, about 50GB, on my memory card. I'm installing apps on the S5's 16GB internal storage and putting all my media (pictures, music, videos) on the memory card.
I should be fine for a while but I can understand how some people would want more storage space. Who knows when those cards will come out and if the S5 will support them. Despite featuring microSDXC card slots, manufacturers had to update their devices to support 128GB cards. Not all of them supported the cards right out of the gate and the current batch may not be compatible with 256GB cards when they are released.
The only suggestion I have, in terms of music, is to not carry around lossless files. You don't need those for portable listening environments. Even 128kbps AAC/mp3 files would be fine for portable listening conditions (walking down the street, riding the bus/train/subway, driving in a car, etc.). That would free up a lot of space if you are carrying around lossless files.