The Stora is a completely separate product and not part of the ReadyNAS family. See the Stora Forum
The ReadyNAS Pro, Ultra 2, Ultra 4, Ultra 6, NVX, are all x86 ReadyNAS. They have the same firmware and same ReadyDLNA. Apart from the NVX, all of those ReadyNAS are 64-bit. The DLNA support is improved over time.
The ReadyNAS uses the open-source MiniDLNA project. See http://minidlna.sourceforge.net/
Naturally MiniDLNA is included in the ReadyNAS GPL: http://www.readynas.com/gpl
The Stora also uses MiniDLNA (you can see the MS2110 (i.e. Stora) GPL at http://kb.netgear.com/app/answers/detail/a_id/2649/
). I would expect that any formats that work with the Stora would also work with the ReadyNAS.
You could well find that your DLNA player is what limits what media formats you can play, rather than ReadyDLNA.