Modern Warfare 2 reacts strongly to CPU performance. The difference between one and two CPU cores is tremendous. Using just one core severely limits graphics performance. The frame rates increase only slightly when overclocking, though, so the GPU takes over as the key performance determinant. Also, it seems there is actually a slight performance hit when using four (instead of two) cores.
Up to 710 MB of graphics memory gets used with AA activated.
To increase frame rates, you really need at least two CPU cores. Using just one will hold back performance.