Jenny...
Interesting question, similar to a problem virt2001 and I are presently tackling. I've never used Dreamweaver, so can't say much about that! But I think you should consider the following: Unless you're displaying your Flash movie in a fullscreen popup window (and, to some extent, even if you were!), designing it at 800*600, will make your movie already a tight fit if the user has his resolution set at 800*600, although it should fit in nicely if the user's resolution is set at 1024*768. So maybe you could still design your Flash movie at 800*600 and scale it down in Dreamweaver, to 90% or even less if it's required till you get a satisfying view of your site in a standard browser window with a tool bar & etc...
Of course doing so & using only one file for any resolution, would mean your movie would be scaled down even more looking at it with a resolution set at 1024*768... And even more again for higher resolutions.
Compromises!
;-)ldnewbie