I'd start by applying the optimal settings in the TCP Optimizer, set the connection speed slider to ~10Mbps.
There is an additional gaming tweak (disable naggling) in the Optimizer's Advanced tab that may or may not help COD4, I'm not sure if it helps that game. I'd start with the optimal settings in the program first.
Overclocking may be a bit outisde of the scope of tunning your internet connection... But I'm sure there are others around who can give you some pointers.
I hope this helps.
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