Dual and triple-core CPUs can't be that very efficient compared to quad-cores. These CPUs are all crippled quad-cores, due to the architecture the cores are disabled but have to run regardless, otherwise the memory-controller could not be accessed. This is a design flaw, hopefully fixed with the next AMD family, so cores can really be shut off completely. So if you have to buy a smaller CPU due to budget constraints go for a triple-core.
The CPUs normally run with about 1.300V VCore which is quite high. Some CPUs will come with 1.250V but all our CPUs run with at least 1.300V. The energy-efficient 705e runs with 1.175V which has shown to save some energy. Let's see how low we can get. Please note, that not any CPU will get that low or some will go even lower. Also make sure you have a board which will support under-voltaging. Just decrease the CPU voltage, don't fiddle with the CPU VID voltage, because it will increase idle-power consumption. We have no idea how that is calculated, but the VID is also responsible for the idle-voltage.
The newly introduced Phenom II X2 went down to 1.180V from it's original 1.325V:
The 705e went down from 1.175V to 0.995V, that's quite nice.
The 720 did well too, down to 1.150V.