It should support EPUB, then -- and calibre can convert DOCX to EPUB. In fact, it has a very nice DOCX-to-EPUB converter that should produce very clean and accurate EPUBs.
I know calibre's user guide lists RTF towards the very end of preferred input formats, but I have never used RTF so I am not positive why, but I wouldn't count it as a recommended format for reading either -- at least, not while there are standardized ebook formats.
