Visit Paul Boag site (
www.boagworld.com). He is brit. designer that keeps a blog on web design, and he has addressed this issue on the past. He also has a regular podcast that you can listen to get you up to speed (they are on issue 58, so you have a lot to listen to, but it will pay the effort).
I agree that CSS and Web Standards are a must if you want to expand your business, and I would also suggest DOM Scripting.
You can also visit
SitePoint : New Articles, Fresh Thinking for Web Developers and Designers for fresh ideas and
A List Apart for more info and articles.
Good luck
Pat