[Mone']

most forum that you might know probably uses css to simply show links that have been clicked in a different colors

<style type="text/css">
<!--
a:hover {  color: #FFFFFF; text-decoration: none}
a:active {  text-decoration: blink; color: #FFFFFF}
a:link {  text-decoration: none; color: orange }
a:visited {  text-decoration: none; color: white}
-->
</style>

you can change the text decoration and color to suit your forum.

just ad it to the header

Mone'