I don't think the P4P8X has a hardware monitor for system voltages, only fan speeds. I had one back in the day, and I only remember seeing fan speeds in the bios setup, not voltage monitoring. You can, however, set CPU, RAM, and AGP vRef voltages, but not monitor them.
I know this doesn't answer your problem, but if you have a microcenter nearby, you can build a new Phenom II x2 build for $200. I just built 2 and one unlocked to an x4 and overclocked nicely. Since you're running a P4, I thought I'd let you know how cheap some of your options are in regards to replacing an old rig.