Agreed, you can get a OC capable CPU, but if the board doesn't support OC then you won't be able to. Conversely the same thing, you can have a OC capable Mobo, but if you dont' have the right CPU then your not able to OC it. Now some cores (like in laptops) are purposefully 'slower' until a demand is set, this is to help save battery power when you don't need to be 'full power' just looking at webpages and such.
Why a XEON? If this is for gaming, you be better served using a i7 then a XEON. The XEON best works for Video/Rendering software (AutoCad, Maya, etc.) not 'gaming rendering', and as a Server to handle multiple requests at the same time