Originally, 1.7 and 2.0 were (are?) different names for the same unreleased version of Celestia. They both refer to the version produced if you build Celestia from the latest source code available on SourceForge. It just wasn't obvious if the accumulated improvements plus the change in user interface were sufficient to justify incrementing the major version number.
Presumably the 64bit version of Celestia currently being worked on will be called v2.0.
Celestia.Sci is based on Celestia but is an entirely separate project run by Fridger Schremp. None of its code changes have been made public.