I estimate power consumption of an OC'd CPU as (TDP + 25%). I realize that TDP is not the power consumption of the CPU, but it is a rough analog. And TDP tends to be a little higher than power consumption. This is pretty close to actual motherboard power measurements.
Motherboard and RAM: 60 - 100 watts depending on factors like type of memory, how much memory, complexity and size of a motherboard. For example, at nominal voltages, DDR3 is about 30 - 40% more efficient than DDR2.
Two hard drives and an optical: 40 watts.
This gives me an estimate that is about 10 - 15% higher than what I measure with my Kill-A-Watt meter. I call that Mr. Murphy's share (ignore Mr. Murphy at your peril ).
One of my systems has an OC'd Q9550, 4 GB DDR2 RAM, a 65 nm GTX260-216, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
CPU - 10 amps, 120 watts (that's my standard estimate for an OC'd quad core proceessor with a 90 watt TDP). And it comes very close to measurements I have made.
GPU - 139 watts, peaking to 168. Call that 12 - 14 amps.
That is 24 amps (288 watts peak) so far. 60 watts for motherboard. 40 watts for drives. Two of the drives in this system are WD Greens, so power consumption is a little less. 10 watts for the sound card. That is an estimated 400 watts (well, 398).
Subtracting 15% = 340 watts. Subtracting the difference between GPU average and peak power - 29 watts - (remember, inexpensive power meters do not respond to short duty cycle peaks, so this comes out of Mr. Murphy's share) = 311 watts.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.