OK, FWIW I decided to run a test here at my office using my desktop to determine the peak load I would pull.
My work desktop is one that I built a little over a year ago. It is a Core 2 Duo 6600 @ 2.4Ghz, 2Gb RAM, a Radeon x1650, 2 WD Raptor X 150s, 1 DVD-Rom, 1 DVD-RW, and a 4Gb iRAM daughtercard. Runs Vista Business 64 with all the bells and whistles enabled, plus Office '07. Resolution is 1680x1050 high color to a 21" FP.
I went back into the server room and pulled a spare 15-amp, 17-outlet
A-Neutronics pdu with LCD output to run my sys through. The A-Neutronics pdu pulls 1 watt by itself. From cold start, my PC peaked at 149 watts while spinning up both HDs and initializing video. Remaining boot time load hovered around 130-135 watts, hitting the low 140s again when bringing up the full screen login prompt and running through post boot procedures, then dropping to a steady 111 watts at idle.
I launched a demo copy of UTIII and entered the first level, which saw the system reach its peak draw at 154 watts.
Is my system high tech top of the line? No. But my point is this: PCs don't draw as much power as a lot of people seem to think they do. Already with two HDs, two optical drives, and 6Gb RAM (4 configured as a fixed solid state disk), I could pull the x1650 and pop in two higher end cards in an SLI config and still would barely clear 250 watt load.
Yes. Don't buy 'cheap' power supplies, however don't confuse cheap with cost rather than quality, and don't equate more expensive as meaning better. The trend towards 550 watt+ designer PSUs is as much a fad as windows and lights inside the case (and in some senses has been exacerbated by it, since you need more power for all those fans and pretty lights).
Some high end PSUs are worth the investment and are necessary at the extreme end, but they are not necessary for everybody and it is foolish to recommend them with a blanket statement, particularly because a buyer could just as likely get suckered on a $100, 550 watt garbage PSU where they could have found a quality $30-40 350 watt PSU.
Bigger != better.