I have one site running on WordPress and I'm in the process of moving my 'old' HTML site over to WP as well.
Both are news oriented but one deals with full length pieces and the other handles newsletter-type aggregations.
However, I have 3.4 gigs of files on the HTML site that I am slowly building links to from the new WP site as well as adding new content directly to WP.
Yeah, I know it's a mess but, I have a pretty big readership and because of the tens of thousands of links, it gets used a lot.
I'd like each site to automatically notify each other when a new piece gets posted, I'm sure there's a way of doing it.
Check out the sites:
http://www.creative-i.info and http://williambowles.info/wordpress
Bill