MediaWiki editor with instant preview?
Solution 1:
Give Gwennel a try
Gwennel Web is a WYSIWYG editor for Wikipedia and other Mediawiki sites. It allows to easily edit pages without knowing the tedious wiki syntax. It runs on Windows XP, Vista or 7
Do note: the developers mention
Gwennel Web is currently in an early development process. There is a lot of missing features but is perfectly usable for most common edit tasks.