If I make a flash header for my website is it likely that there will be users that can't see it? thanks
You should take care to detect and embed the Flash Player in (X)HTML in a W3C compliant way. Look at this page for details: http://www.hofmanns.net/flash-en.html But there are many usability concerns regarding Flash, because Flash breaks with many of the conventions associated with normal HTML pages. Things like selecting text, scrollbars, form control and right-clicking act in a different way than with a regular HTML webpage.