What's the big deal with having your content in a separate file anyway? Beyond SEO and the single responsibility principle there is what I consider to be the single most important reason: Accessibility.
The site navigation should be simple and you should have a site map. All of these suggestions are SEO best practices, but really you are doing a favor for anyone that is coming to your site that relies on a screen reader to navigate the web.
Once you've reached this step you can do two more things. You can build a simple CSS for people that want to print your site. In this print version of your CSS turn off the navigation and anything that is not content.
Next add a little bit of flair with CSS to your site and make things look better than 12pt black Times New Roman on white background. Throw in a little color and some font sizes here and there. Use this as the WAP version of your site.
Once you've finished with this, you can finally go to the holy land... Silverlight. This is when you can turn on all of the bells and whistles. You will need to invest some time in writing an HTML to XAML converter, but it will be worth it. When you launch your site and everyone on the Internet can enjoy your site you will have a site worth being enjoyed.