It depends on the game, some games can use more than 4 cores, but most can't even use 4.
BF3 multiplayer can use all 8. I think that Stalker: Call of Pripryiat can only even use 1 core.
If the game can only use 4 or less, then they will perform the same.
It is the same with the x1xx and x3xx chips in this regard.