More often that not, a website can benefit from using a text-based navigation instead of an image-based navigation. It's commonly understood that words that are placed near the top of your code (page) are given slightly more consideration and weight by the search bots in determining what your site is about. Because the majority of websites have their navigation across the top of the page or down the left hand side, placing important keywords in your navi and ensuring they are 100% crawlable can help boost your optimization efforts.
Don't know if you're navigation is text-based or images? Take your mouse and try to click and drag over the text within your navi. Can you highlight the words? That's text-based. If not, you're dealing with images. Of course, you can add Alt-tags to your images to help the cause (especially if you can't replicate the look and feel of the navi with plain text), but there's still no substitute for using plain old text links.
Generally, if you are hoping to improve your SEM for a given set of keywords, those keywords can and should be used in your main navigation. You have a home page that is a catch-all area that then directs visitors to the categories and main themes of your site. Those categories/themes should be highly visible pages within your domain, and the best way to make them stand out, pass on optimal benefit through internal linking, and make your site climb the SERPs is to drop them as text in the main navigation.
Go on, give it a try. While there's still a good 50 little odds and ends every website can run through to ensure their on-page factors are optimized for the engines, this is a good start and a practice that most of your competitors will be overlooking.