AA member rj1307 has released a new SD cartridge solution for the Atari 7800 that he calls Dragonfly. This cart is able to do some of the functions that were intended for the XM module (which is now on hold given Curt's passing).
For correct configuration, the cartridge uses .a78 files and the header. It supports 4k, 8k, 16k, 32k, 48k files - without banking, 64k, 128k, 256k, 512k with banking. In addition, 16k RAM for $ 4000 and POKEY for $ 4000 and $ 0450. Various combinations are possible. Also supports Absolute and Activision.
The cartridge can be ordered with POKEY and Yamaha chips for enhanced sound. Ordering information can be found here: "Dragonfly" my version of the Atari 7800 SD cartridge
Here is a video reviewing the Dragonfly and how it works.