I just took a few minutes and measured the actual "CPU VTT" voltage with DMM to compare it with the BIOS settings. Here are the results I got on my board (BIOS 11/07)

"CPU VTT" BIOS Setting | Actual Measured Voltage
1.20v | 1.098v
1.25v | 1.147v
1.30v | 1.171v

I then set my "CPU VTT" to 1.20v in BIOS and measured the actual voltage under heavy load (Prime95 Small-FFT's)

1.20v | 1.098v (steady, the voltage didn't budge at all)

I hope that helps a bit.