It's dubious whether your real power draw will exceed 200W. A fairly loaded AMD dualie pulls 250W absolute max peak (momentary) -- long term, maybe 235-240W. I was there for this: http://forums.silentpcreview.com//viewtopic.php?t=2580
Those power number above represent the AC power daw
-- the DC power translates to ~175W max if you assume 70% AC/DC conversion efficiency in the PSU.
For another reference, a P4-2.8 test system I sometimes run -- with 512K ddr, 1 HDD, 2 PCI cards, 1 VGA (AGP), CD drive -- never
exceeds ~165W AC power draw with a Nexus NS3000. That's 115W in DC. The PSU is coasting at 40% of rated power.