browsers and search engines generally don't care what extension you use. back in ... 98? 99? when I last used .shtml pages I never had trouble. I doubt things have become worse.

If you're serious about web development, I strongly recommend learning some server-side scripting language - and I'm a big fan of PHP myself. Of course, that doesn't mean you can't go to the SSI for now, and just switch over to a PHP include() of the same content later, followed by a complete database driven template system (which isn't always the best solution anyway) eventually.