I'd recommend using the HTML validator plugin, since you are using FF (though I don't believe they have an OSX version, if you are using it). You'll notice that anchor tags shouldn't have alt attributes but all images should. If you want text to pop up when hovering over something, that's what the title attribute is for. You also may want to consider what audience you are reaching. For my sites, I have noticed ~5% are still using 800x600 resolution, which does not work well with your layout. I'd definitely redo the navigation links in as organized a manner as possible. You want all the really common/necessary ones to be on the front page. Divide the less important ones into other sections, or you can even have dynamic menus that show the links when you click on them.
The site does have a very peaceful and soft atmosphere, which is very fitting for what the content is. It's good that you are moving away from tables (as they should only be for tabular data and not layouts), but you don't always need divisions. If you are just modifying one paragraph, you can stylize that p tag specifically.
Hope something in that rambling is helpful!
Check out Gordaen's Knowledge, the blog, and the MR2 page.