I had to upgrade a lot of the firmware in my son's system in order to run COD 4 on Vista x64. That includes the North/South-bridge chipset, the GPU chipset, Audo Processor chipset. Once those updates were done some of the drivers also needed to be refreshed. Additionally, I applied 43 fixes to Vista itself.
With all of that done - COD4 single User and multi-player both run like velvet!
His system includes:
D975XBX2 mobo
Q6600 Duo Quad
Nvidia 8600 GTS
Integrated SigmaTel Audio Processor
WD 500GB HDD
ThermalTake Liquid Cooler
8GB DDR2