First... 99% of the games currently out don't require more than 3GB of RAM, at most to run efficient. The reason you see most systems with 4GB or 6GB is the best utilization of the ram slots and memory combination's. Most RAM comes in 4GB (2x2GB) sets or 6GB (3x2GB) sets to allow for better use of dual or triple channel memory.
As for "benchmarking", you will see most systems maxed out to provide the optimal performance. You won't see someone benchmarking a new GPU using a single core P4 and 512MB of DDR RAM. You'll see them using an i7-980X with 12GB of RAM, which will of course show higher frame rates!
Second... Based on your question, RAM usage doesn't get effected by an stock or overclocked CPU. It is driven by the application and what it needs.