> So, what is being shown if an 404 error occures? 

The original page, not rewritten (therefore images embedded 
won't work)
Use "no error pages" to avoid that

> Then I like it better to create the 200 page because also 
> my offlince cd users can see the error page (otherwise 
with 
> no internet connection they see probably nothing but a 
> Browser error).

This is a very bad idea, and breaks HTTP rules.
Some idiotic browsers such as Internet Explorer replace 
error pages from upstream servers by their own page if the 
page size if smaller than a ceryain amount of bytes (this 
is crazy..)

Try 302 redirections with a 200 page at the end - this will 
be a "less bad" solution