version 1.5.6.1 or 2.0

Is there some news about the release date of 2.0?

Not that I know of.

napok, since you have a stable store, you can safely leave that running while you tinker with an exact clone of its fileset in a new directory for the clone, and an exact clone of the database given a new databasename (in phpMyAdmin you can make a copy of a database having a new name, in one button poke). If you wish, you can set up several of those and try upgrades to different levels (1.5.5.1, 1.5.5.1 CE RC 1 or pending RC2, or 1.5.6.1 (skip 1.5.6.0) -- beta and then production 2.0 are still pending, so not yet options to try.