Use the search engine here. You'll find the P4S8X is garbage. For instance:
Although Asus claims the board supports DDR400, the use of this memory at full speed will likely cause reboots, errors, general instability.
Although the Chipset supports AGP8x, this board won't run stabley at AGP8x on many people's systems. More crashes, etc.
It appears to be a problem with the 648 chipset, and most likely related to the memory controller. Gigabyte is the only company to address these issues, in the 8SG667 and 8SG800. I'm not certain what Gigabyte did to stabilize their board, but evidence points toward a slightly higher chipset voltage.
Like I said, use the search engine. You'll find dozens of complaints about this board. Many people had to change video cards or boards to get their systems to function.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>