Actually the first calculator looks pretty correct for power draw.
Most
PSU's ratings are not that close to real world. Most put ratings for a temp of 20-25C. Real world would have that
PSU sitting at about 50C at a minimum. That degrades the performance. So yes, while the system draw may be only 200-300W. The peak draw at startup, and
PSU degradation due to heat may mean that 250W continuous draw may need a 600W rated
PSU.
__________________
Silverstone ST405, AMD64 3500+ Winchester, Gigabyte GA-K8NXP-SLi,
4x512MB Generic, Palit 8600GT Super, 120GB WD SATA, 2x 320GB Seagate, Asus LightScribe DVDRW