At my now dead startup Nisvara we where building Silent computers using passive cooling, and figure out how to build server rooms that didn't require air conditioning just outside evaporative cooling towers.
Our designed cooled each component individually so we did profiling of each components power draw.
I dont' have the numbers in front of me, but if I recall for a typical P4 3Ghz system we saw the total average power consumption at something like 75 watts when idle and 150 watts or more under load. With the ACPI on it would drop considerably when idle. (I am not including the monitors that also draw 75 watts or so)
Again we were more interested in were heat was generated so we measured power dissipation per component which for all purposes is directly equivalent to watts used.
We found watts for a component was quite different then the watts on the power line.
Why? Because in this breakdown almost 40 to 50% of the power was lost in PC's power supply's!
Both main and on the motherboards on board supply's needed for the CPU and chip sets.
This was very high since most PC power supply were only 60% efficient!
So all loaded inside the PC show up as almost 2x on the 110 volt power line.
So of the peak 150W coming in what's left after being stepped down is a remaining 80W or so.
Hard Drive 12 watts assuming 1 80Gb Maxtor DiamondMax.
North and South Bridge, 1 to 6 watts
Support chips, almost 1 maybe 2 watts, things like the NIC and other support components were insignificant.
CPU which could vary from 20 watts to 100 watts depending on it's load.
Running like CPU burn, CPU test or CPU stress would max out the CPU's power, again with the power supply low efficiency an 80 watt increase in CPU power use results in an 160 Watt increase on the 110V power line! We didn't not expect this when we started.
If you add a high end graphics (Nvidia/ATI) card then add on another 40 watts 2x so 80 watts on the power line.
Another interesting thing was 10 watts for fans!
Here is another unexpected result, the hotter the system ran the more power each component draw. This could add another 10% or so. So a cold system like just after power up uses less then a hot one.