Joined: 12-February 03
Member No.: 23581
Xbox Version: none
360 version: v3.0 (falcon)
"ATX 12V" has an additional 4 wire plug (2x 12V + 2x GND) that supplies the CPU and other voltage critical components on the mobo with 12V on its own rail (to ease the draw of 12V off of the 20 or 24 wire main power connector, which then lengthens the life of the power supply). A standard ATX power supply doesn't have this 4 wire plug, and isn't meant for modern mobos.
There's another spec of ATX power supplies that uses an 8 wire plug (4x 12V + 4x GND) instead of a 4 wire plug.