Master Matthew is a loving idiot, but he's not entirely wrong in his sentiment, only mostly wrong.
Multi-Processor support was only available with Source 2007 (The Orange Box) after a large refactoring of the codebase, and it wouldn't actually be stable until the Left 4 Dead branch. While rendering is technically done on the GPU, the CPU is still responsible for communicating and assigning tasks and data to the GPU, and having more cores means less load as the game also needs to do audio, physics, input etc on the CPU.
If you could give me your PC specs, that would help. Are you running on a laptop? Are your Source games using the correct GPU (many PCs have the option in your GPUs configuration settings to use either the integrated or dedicated graphics system, and most software is assigned the former by default which can affect game performance negatively)?