Underclocking / Undervolting possible?

The following site lists some aopen mainboards as underclockable:

http://www.silentpcreview.com/module...rtid=69&page=1

Does anyone know, whether it is really possible to underclock /
undervolt a pentium 4 system using an aopen mainboard? Could I, for
example, use a pentium 4 (2 Ghz) as low as 1Ghz? Could anyone
recommend values for vcore etc. to go with that?

Any help would be realy nice. I've already tried to find infos on
this, but there is not much info on that topic online.

Regards, Daniel