I was pretty much able to pick up where I left off a week ago, and start getting a good grip on CSS.
It's pretty amazing how having each page reference a style sheet can add so much flexibility and streamline the design process. I can definitely see how this will help in the retooling of all of my sites, plus any projects that come my way in the future.
After some correspondence with one of the experts in accessibility, I have decided to forgo the HTML vs. XHTML debate for now, and stay focused on learning XHTML. Down the road HTML 5.0 will make itself know, but I need to learn the basics and focusing on XHTML is the best course for me now.