To back up a site's content, is that a matter of downloading a single xml file? 

EDIT: I noticed I inadvertently put this in the 2007 thread, would the same answer apply to the 2008 version also? 

yes (sort of), to backup the content it is just the single xml file however there are other xml files that are used for other purposes such as users are stored in the /files/_users folder and mailing lists in the /files/_mailinglist/ folder etc... so if you want a full backup then you need to copy the entire /files folder off the root of your website.

Thanks, I also found the answer to the question in another thread concerning multiple sites upload.

At the risk of asking another answered question, how many single page articles can the system realistically handle? Not all of them need to be in a menu, but could be found by a site search. More than 2000?

i'm sorry i do not have any metrics on this matter but tandem server will be able to handle a few thousand pages/articles realistically. it will depend on your server but in general you should not have a problem.