Hi,
Firstly I want to say I like the concept. I'm always looking for these types of tools.
When a think of a mortgage calculator web site, I think small. Something compact and easy to use. I'd suggest getting a fixed width layout, not fluid. Get a logo done or at least change the colors. The slogan blends into the background and is hardly noticeable. Your navigation links are squashed together and don't stand out. I didn't even notice them over on the right until about 30 - 60 seconds in.
I won't comment on the tool itself as I haven't a clue on mortgages. I do suggest Javascript functionality, for a calculator people don't expect the page to refresh for the result.
Good luck
Richard