I don't know of any such software and I'm pretty sure it doesn't exist because it would require some form of embedded current sensor.
Theoretically speaking, if motherboard manufacturers integrated a current sensor in the motherboard power input buses (somewhat like the embedded temperature-sensing diode), it should be a simple matter to develop software to measure power consumed by all devices that draw their power requirement from the motherboard, including RAM, CPU, graphics and other expansion cards, USB devices, keyboard, mouse..... Even then, it still would not account for those devices that take their power directly from the PSU such as hard disks, CD/DVD drives, etc.
As things stand now, the only way to measure power consumption would be to measure the current flowing in each PSU output line, multiply them with the respective voltages, and add them up. A rather complex operation. A somewhat simpler method would be to measure the AC current at the INPUT to the PSU.
A post script to my earlier comments that a good 300W PSU is enough : It's true, but don't get me wrong - I'm not against anyone getting a PSU of higher rating. A 400W PSU for example, will leave that much more headroom, especially if you have tons of RAM and several hard disks running. What I meant was that a good 300W PSU that can actually deliver 300W is perfectly adequate to run the more usual single-HDD config with a 6600GT.