The core clock really doesnt effect overclockability, unless you mean the lower the core clock the more headroom the card may have. I think the base for a stock 7950 is 800mhz. Anything above that is considered an overclocked card. Both of my cards have different core clocks, one is the OC edition with a core of 900, the other card is a boost edition with a core of 800, but boosts up when needed. As far as overclocking, both seem to be able to reach the same with no problems and I run them at 1100/1400 when I need extra power.