Stylesheets would be great, but since some of the site contains Flash, which as far as I understand, you don't want to show for slow connections.

You might be able to share some of the site design and just replace the HTML code where the Flash is with something else.

There are a couple of rules of thumb that you might do to improve the load speed for slow connections. Check out this link at the Yahoo Developer Network.

They have a set of good rules that can help you speed things up which might not be very noticeable for people with broadband but will be greatly appreciated by your dialup users.