CPU-Z reports the core voltage correctly on the DFI Dark series so you don't have to wait for anything. Get overclocking like everyone else is doing. Keep the reported core volts under 1.40 at full load and you'll never have a problem.
Here's a nice review over at Legion Hardware showing you what sort of results to expect and how much voltage you'll need when overclocking an E8400.
On the box of my shiny new E8400 it says the default voltage is 1.225 so the 1.23 volts that your motherboard is giving your E8400 is a perfect starting point.
