Take a look @ some benchmarks. It's the video card(s) that make most of the performance difference in games, generally speaking. And the CPU's in-game purpose could be becoming less and less it seems...
In correction to my previous statement, modern GPU's also handle much of the geometry calculations, hence why we have Vertex
pipelining artichitecture; a vertex defining a point in 3D space (X,Y,Z) in relation to the origin
Also, on the horizon we have the Physics Processing Unit, or PPU. This will offload calculations such as collision detection, gravity, moveable/destructable objects etc that is a CPU hog right now, especially in physics-heavy games like HL2. Take a look @ these tech demos which give an example of what can be accomplished with the PPU: http://www.airtightgames.com/currentproject.html
Honestly, it won't be long before we have dedicated AI processors...