Well with your navigation, you have empty links:
<a title="Contact Us" accesskey="c" href="contact.php"></a>
You then fill in the gaps with background images which are just solid blocks of colour and text, these could and should be easily replaced with two background colours and some text. The text should also be resizable for partially sighted users.
You should also have variable sized fonts for the whole of your page otherwise the affore-mentioned partially sighted users may not be able to read your site content. This means specifying font-sizes in em's or %'s.
Don't use images for spacers, just use some bottom padding. Also every image should have a proper alt attribute, not just alt="".
You have used lists for your navigational links, but then for your text you have not used paragraphs. Semantic mark-up all the way!
Hide your CSS from older browsers that won't understand it by including it between some <style> tags:
<style type="text/css" media="all">
The last thing that I can spot is try using XHTML 1.1 and send it as application/xhtml+xml, it's better than normal XHTML (for reasons that I've forgotton right now) and it's just as strict, if not more so, that XHTML 1.0 Strict.
I also find irony when you call yourself Flexible Web Design and have a fixed width layout.
Try an elastic layout.
These are just the things that I spotted, I'm sure there are more that I missed but I'm sure that someone else can pick up on those.