can't be specific without a link to your site...but how I did it on my theme (cuz I wanted to have a little shadow on the line, just a bit fancier than standard css) was to see how wide the content area was in my css, then draw a line a bit shorter than that (in GIMP, or photoshop or whatev)
then under the line of code that calls the title (in index.php) add:
<div class="lineBreak"></div>
and in css
.lineBreak {
background:url(images/line.gif) no-repeat;
}
there's a few ways to go about it, this is just the way that worked for me.
you could just wrap the title in a div too, with the bottom border, depends on exactly what you want it to look like.