Diffrent card yield diffrent overclock, even on same batch and same brands. Thats the nature of overclocking. Sometime you get lucky can get higher clocked without much trouble and sometime you wont. My X1800GTO can only reach 648MHz core and I see someone even reach over 700MHz stock and reaching 800MHz with water. I can say the same with my card even with crazy cooler and more than enough power supplied through the card