Perhaps true for the general case, but not true for people choosing to mine for Bitcoins. An single efficient modern GPU will use most of that 300W by itself when running full out.

http://www.bitcoin.org/smf/index.php?topic=7692.0

And the motherboard uses 10W, the disk 10W, and the CPU 50W. We're still not at 700W.

True. I wasn't arguing for 700W with single card, but felt that 300W sustained might be a bit optimistic if one is aiming for $500 of Bitcoin revenue per month. The OP was offering a conservative estimate, and one could just as well point out that 20 cents/kWhr is also on the high side.

I was just using the layout my tech guy at the comp lab told me, and he was talking about using two gpus.