Welcome to the forum.Great topic. I'd love to hear more users' wish lists.
I realize that some people prefer not to say wishlists in a public forum, so if anybody reading prefers to send wishes private, you can e-mail me at webmaster@ebookcompiler.com
A great technique BTW I've been using a similar technique on one of our web sites, for about 2 years. Your version is a little different from my version.
What I did in mine was start off with the 640 version (which is actually the index page), and on that page redirect to the 800 version if the user's screen is big enough. I did it this way because if the user has JavaScript disabled or whatever they still get the 640 version. You could consider this.
I also *think* that I might have an idea for another solution. Let me think about it, and do some experiments.