If you have an ATX supply, you have 10s of amps of well controlled 5V DC. Why not just use that directly?
I currently have a '328 running off the stand-by 5V of an ATX. The Ardu' grounds the control pin of the ATX when it wants to fire up a massive LED light (well, only about 20W worth but still more than stand-by power), draws the power it needs and then turns the ATX back to stand-by again after. The 5V supply is entirely stable enough in spite of PWM control of 20W of LEDs. Arduino's on-board regulation not required.