Just curious why those models? Are you buying them new or used somewhere for a good price?
As mentioned above it will depend on the game. If it is a CPU heavy game or one that is optimized for more than 2 cores, you will see it holding you back more than other systems. Bad Company 2 is one of these. It really hammers a dual core but a quad core runs much better. It seems the more CPU you have for that game the better. There are others that are the same, and also others that really are not CPU heavy at all. There is alot of Physics going on in BC2 with the explosions and such that really put a load on it. IF you are playing Modern Warfare 2, its a DX9 game without much going on and you will be more than fine.