Hrmm, im curious to see how you came up with your numbers.
I spec'd a system with an i7 860, (6) sticks of ram, (2) 10k rpm drives, (2) SSDs, (1) 5970, (2) dvd burners, (2) 120 mm LED fans, (2) 120 mm fans, (1) PCIe - 4x card, (2) usb ports requiring power.
Configured for 100% loading, which means everything is running at peak capacity and calculated a total of 512 watts.
Assuming overclocking adds a max. of 100 watts (not likely at all) then peak power wiould be 600 watts. I don't like to run my PSU's over 80% of max capacity so that would require 720 watts. A 750 would do, but then again you get power drop at higher temps and while the 80% mentioned before should cover the efficiency rating and some power drop I would throw in another 100 watts which should give me a longer lifetime for the PSU. 850 watts will satisfy any need for a single proc. computer with (1) 5970 including overclocking. You could even run (2) 5970's quite easily and bump their clocks up a bit as well.
As far as the waftability - well a rolls royce, cruising down the street, garners the attention of the social elite (ie. hot girls) who want to go for a ride in your ride. It also gets you a lot of free grey poupon mustard.
A 1200 watt power supply waftability gets you the attention of some fantastic dust bunnies.
Also please keep in mind that 1200 watts is getting close to the maximum allowable load on a 20 amp circuit (which the ceiling is 1600 for new construction). Most homes (and business for that matter) don't have a dedicated home run circuit for their computer alone. They share the 20 amps with whatever is connected to the circuit. (haha luckily i did 2 dedicated home runs for my office when i finished my basement, hows that for geek creds?)
In any case while a 1200 watt PSU is nice to have, it's not really needed...