I dunno, I've had REALLY BAD LUCK with OEM cpus. 4 out of 4 OEM-Celeron366's wouldn't hit 550. 4 out of 6 Retail Celeron366's did 550 at stock voltage. I know this isn't enough for numerical scientific proof, but it is enough for me to go retail.
FWIW - 5 out of 5 retail 700E's hit 933mhz. Only one of them would not go over 1000mhz. I say retail all the way for maximum overclockability, an extra fan to put on your chipset, and a 3 year warranty.