Managing Content

There's a project that I've been 'developing' for over a year now, and I'm looking to push on to the final stages, which includes getting the web site filled with content, and ready for use.

Basically, I've had a fully coded XHTML/CSS design created, but, at present, such is only one page. I'm now wondering what would be the easiest way to implement the design into a manageable system, whereby if I need to update one element of the design, I don't have to do so to all pages. I've worked with SSI previously, but I'd be interested to hear what else is available, or what you can suggest.