If you are considering designing or redesigning a web site, you may want to read up on Theme Pyramids, an SEO friendly site architecture. It's not a new idea, but I've used this site architecture on a huge PR7 commercial site that 85% of US households know about, and the results were phenomenal.
It's a great way to improve rankings (especially at Google) by improving the flow of relevance around the site. It is also a great way to increase the number of indented listings on your site. Converting an old site to Theme Pyramids will mean rearchitecting your site... It's a great time to fix lots of things about a site like moving to search engine friendly URLs if you don't already use them. Regardless, it's going to mean resturcturing your site so URLs are going to change... so you'll need to familiarize yourself with how to redirect web pages with 301 redirects.
Hope you find it an interesting and useful read.