Firingsquad recently posted a how-to to help determine your system's power requirements when shopping for a new power supply - see http://firingsquad.gamers.com/guides/power_supply/
Using their figures (some of which come from the AMD guidelines), a 2.8G P4 with 2 HDD, sound, DVD and CD-RW, + 2X512M ram requires 17.3A on the +12v rail at max draw for all devices. They suggest adding 20% for expansion and to cover PSU fudging in manufacturer ratings, giving a need for 20.8A. Very few PSUs offer this much juice on that rail - the only quiet one is the Quietpc 460 Watt.
I wrote to ExoticPC.com, expressing interest in their new "ultraquiet" Silenx(14 and 18 db) modded Fortron 400W PSU models (rated at 15A for 12v), laying the above out, and here was their response:
"Those values from Anandtech are very exaggerated numbers far beyond worst case scenarios. There is no possible way a P4 2.8 processor can actually draw 10.1 amps of power on the 12v line. The P4 2.8 processor has a maximum power draw of 68.4W not all of which is used up by the 12v line, so more realisitically, it will only consume approximately 3-4 amps max on the 12v line. As for the hard drives, your typical 7200 rpm IDE hard drive consumes 500mA max, so two of those would be 1 amp (even two top-of-the-line 15000 rpm drives will not consume 4 amps, the fastest Seagate 15K drive uses 1 amp max). That's 5 amps so far, add CD, DVD, motherboard and fans to top out around 8-9 amps max, which is well within the maximum steady current the 400 watt SilenX PSU can handle. The 30% peak power delivery [you asked aobut] is a buffer on top of the 15 amps the [Exotic] PSU can provide for a total of 19.5 amps. It is capable of delivering this amount of power for 30 minutes, this is just a margin of safety you have in addition to the 15 amps to make sure your power supply doesn't die out just because your PC is exceeding the 15 amp max. "
I would be appreciative if anyone with knowledge in this area can comment on this power requirement discrepancy here. Firingsquad's figures do seem "out there", but on the other hand Exotic is looking to sell their units....I am aware of MikeC's power draw tests on his own systems on this site, but by today's standards those systems are only midrange or lower so their applicability to high-end, overclocked setups is very questionable.