I've recently completed doing almost exactly what you're up to, except I was using mostly 'include pages' rather than borders.
Main advantages of using frames for me were:
1) The content page was almost all content, very light on other things that could distract a search engine (and also loaded very, very quickly).
2) My table of contents frame for each section of the site could be quite extensive since the page only had to load one time within the frameset.
3) I could take a page from any folder anywhere and stick it in a frameset where it was needed just by including a link in the table of contents for that frameset.
Now, the problems I've had converting to a 'flat' format:
3) Those pages that served double duty in different framesets are now giving me a problem. If I use the 'Widget A' page in three different sections, which table of contents do I include on the page? A general one where the user might feel lost, or put a separate Widget A page in each section? I'm still fighting with it. I have to restructure much of my directory scheme to more closely match the navigation structure.
I don't use Frontpage borders except for the bottom footer on each page and can't remember how it handles the left and right borders. Do you need different borders for different sections or will one border suffice for all pages? I mostly use the 'include page' function. I find them a bit clunky to use but it is doable. This thread [webmasterworld.com] touches a bit on it.
To sum up whatever the heck I said: The page size is going to change as will the text to code ratio. Hand-in-hand with that is that you may have to rethink your overall navigation structure and how you have your files and folders ordered.
Did I make any sense?