You can use border-radius. However, this isn't standard CSS and you will have to call on either moz or webkit to do it, therefor it only works in Mozilla/Firefox and Webkit browsers (Chrome, Safari etc.).
HTML:
-moz-border-radius-topleft: 5px;
-moz-border-radius-topright: 5px;
-webkit-border-top-left-radius: 5px;
-webkit-border-top-right-radius: 5px;
Just insert in your div properties.
However, there is always the option of using images to rounden the div.