IMO when building websites you have to just say "I can't help everyone all of the time". In this instance, most of the public don't know about resizing their font using that feature, so its mostly not a problem.

As for the 800x600 fixed width, if you are convinced you want to do this. Then I would make the fixed width of the website 750. Because some browsers have odd things on the left and side hand side of the screen including the scrollbar. Depending on the content of the websites a lot of times your can set the width to 100%.