Ok a pre over clocked card is guaranteed to run at the over clocked rate. These days its a lot less common but it is possible to buy a standard card that wont clock as high as a factory over clocked card or just not over clock at all.
A stock card used to be able to over clock as high as a factory over clocked card but these days with the inclusion of voltage tweaking on most factory over clocked cards as well as some standard cards its the factory over clocked cards that usually end up out clocking the standard cards.
The reason for this is that the voltage regulatory circuitry and the cooling are all of a better quality and design meaning the cards can run more stable power/voltage and keep the cards cooler.
The MSI Hawk cards are the king of this with a lot of the components being well above the usual grade and are classed as military class. This would be my first choice with the Gigabyte card second.