By using the DOWN+POWER procedure, all memory contents will remain INTACT. Dual-boot included.
Firmware? Use the 1.20 version, either oficial or custom works well. I, myself, use the BR Firmware since im brazilian. Just search around and you'll find a nice firmware for ya.
About dingux pack, you can use Santino's v0.35 LocalPack. Not all apps on it are fully updated, but most works well.