It depends on how many apps you use, pictures you take, music on your device, etc. I have a Lumia 521 with a 8GB Micro SD card, which equates to about 16 GB. I don't intend to get a larger SD card, although I would like to. 8.1 Hardware won't come out until the middle or end of the year, and I doubt it will be a big upgrade (probably something like the icon with software buttons) or even come to T-Mobile.