I would answer truthfully and full of information if I could, but I will not have my hands on Dreamweaver CS5 until after they start shipping it.
You might have a look at specs in the product section, and see if the specs have changed in a favorable way for you, regarding the 'memory handle'.
Regarding whether upgrading from CS4 to CS5 is worth it for a web developer, I would say yes, it is. I think staying current with software is the least you can do, especially if you are going to be working with the suite in any way at all. We have already seen that there will be more support for people who work with PHP (though I don't really know what that will involve), so it stands to reason that it will be value added to you.
I don't know about your first 2 questions. CS5 hasn't released yet.
Is the upgrade from CS4 to CS5 (web developer) worth it?
See David Powers' post:
The documentation for CS5 is available here - http://help.adobe.com/en_US/Dreamweaver/CS5/Using/index.html.
I have not heard anything about the ftp support, but the set-up has been improved I believe, (or at least according to the documentation).
As for the upgrade being "value for money", personally I would have to say no if you already have CS4 as I cannot see any real improvements over my current programs. But if you are using a cms system that cs5 supports it may well be, although it should be noted that the inclusion of the "browser widgets" module, (extensions already available on the dreamweaver exchange) may be a significant improvement for those using the spry framework as it now includes things like a spry lightbox.
You may also be interested in - http://www.adobe.com/devnet/logged_in/sfegette_dwcs5.html.