Crysis doesnt use more than 2 cores. anything U3 does and COD4 doesn't either.
COD4 -
http://www.guru3d.com/article/cpu-scaling-in-games-with-quad-core-processors/3
Crysis -
http://www.guru3d.com/article/cpu-scaling-in-games-with-quad-core-processors/9
To be more precise I've seen no real gains in any games by going from dual core to quad core outside of running low resolution benchmarks which don't account for much in the real world. If I were to go by that data I've seen games that benefit from 8 cores. The picture changes quite a bit as you increase the resolution of the games. in which case any Core 2 Duo or Core 2 Quad over 3.0GHz or so won't generally see massive gains in CPU speeds. It's all GPU bound at that point.
I've seen some gains in some games at 4.0GHz on Core 2 Quad's and Core 2 Duo's but usually the difference isn't going to make something unplayable on a given card suddenly playable if that's the only variable altered.