512Mb/1Gb DDR3 for ReadyNAS Duo v2

RAM upgrades are not possible on the Duo v2.

Buy an Ultra 2 (RNDU2000). This is more powerful and comes with 1GB RAM from the factory.

Has ReadyNAS Ultra 2 new firmware/interface as ReadyNAS Duo v2?

No the Ultra 2 currently has pretty much the same interface you're familiar with on the Duo v1

MaximG - Not at this time. They have hinted that it may be ported to the x-86 units in the future but have not stated so definitely. They have basically said it will NOT be ported to the Sparc based units (Duo v1/NV+ v1). The numbering (5.3.x) on the ARM based units leaves room for it on the x-86 (currently 4.2.x).

Be advised that the Duo v2 lacks many features found on the Ultra 2. For two, it does not currently support NFS, although there are hints that it may come in the future, and it does not support rsync (no hint about the future). Many 3rd party add ons for the x-86 units will not run on the ARM based units. If you plan on keeping the Duo, with the Ultra 2, you could set it up to automatically back up on a schedule to the Duo via rsync. (Yes the Duo v1 supports both NFS and rsync). The first backup should be done with NFS and then switched to rsync for each subsequent backup. It basically synchronizes the files by share between the two units. (My backup jobs run every night).