for high end PCs with modern components and specially new generation graphic card SMPS affects the stability of the system. Its not always the wattages but Ampere rating on the rails thats more important.
My 400Watt unbranded PSU couldnt keep my PC stable enough with 6800GT, lots of reboots, PC hanging. Replaced it with good branded 450 watt with higher ampere ratings and everything smooth and perfect.