Guys, the full stock settings for a reference design HD3870 are:

2D --> 300/1126 1.241v
Low3D --> 300/1126 1.241v
High3D --> 777/1126 1.327v

And usually lowering 2D voltage in BIOS ends with higher 2D voltage, dunno why. Power consumption is more or less the same, even if you use 3D voltage with 2D clocks the chip sucks +-2W. RV670 power consumption scales mainly with frequency, not with voltage. You can test it if you want, looking at temperatures and wattage to confirm it.