You don't need an SDHC card. microSDHC is 4GB and bigger, smaller are just microSD. Be sure to get a microSD to miniSD adapter.
The naitve firmware and dingux are independent on each other.
You can tell the version of native by going to system settings and about.
The link you sent leads to dingux-7800, which is just an atari 7800 emulator for dingux.