CSS and not only can you get a roll-over effects like JS, but you can also set your menu at the bottom of your mark up (HTML/XHTML page) so SE's see your keyword-rich text first.
If you take a look at my site (
www.3wows.com) you can see the menu is at the top, then look at the code - it's at the bottom.
There is a problem with CSS menus and IE, where a blank appears for an instant on mouseover, but this can be worked round by making the background that appears near the color of the swapped image/color.