go to bios, you say you cant find the 'voltage menu'? on one of my machines there is something called a "cell menu", in there is where i can adjust my "CPU FSB frequency"... try that.
if not look in every menu (you don't want 'ram frequency') if you cant find it in a menu then try to "load optimized defaults" (who knows, maybe it wont give you a 'voltage menu' unless you do that)
if that fails.... your SOL...
no just kidding, its in there somewhere.