I ran a 366 celery(PPGA) on an Asus P2-99 for 2 years. It ran at 458(83Mhz bus) with just a crappy fan and heatsink, no thermal paste, no voltage bump. It ran for short times at 550(100Mhz bus). So with a good fan and heatsink and more voltage, 550 easy, 600 probably.
-doug