ACTUALLY, in my experience(only with eVGA 7900gt's, other companies may differ) the factory OC'ed cards use slightly better cores than their non OC'ed compatriots allowing for slightly higher overclocks. For instance my 7900gt goes up to 625/1950 with gpu vmod and vmem mod. the core will go no higher without artifacting. My firend's 7900gt CO however will do 685/1950 with the same volts and same cooling system (vf900). This seems to be corroborated by two things. One i have noticed much better core OC's on the CO and KO series than on vanilla ones. Two, i was talking to the head engineer at eVGA the other day (long story on how that happened) and he was telling me that the way they find the KO and CO series is by simply setting them at that speed with the stock sink and run ATItool art scanning on em until they fail. I think he said if they can run for 8-12 hrs or more then they go into the good GPU bin (aka the factory OC bin) and if they run less than that with no arts they go into the vanilla bin (aka 7900gt vanilla, no factory OC). THis would expalin the higher OC's commonly seen on the factory OC cards. they simply use better cores. soooo, in conclusion i would say spend a little extra on the higher OC one and u are very likely to get a better core. just my $0.02
Morgash