According to our tests, a dual-core CPU is sufficient for Avatar, while a single-core CPU wouldn't be able to keep the graphics card fully utilized.
Despite essentially the same load being distributed over more cores when using a quad-core CPU, the frame rates increase significantly. This is not the case when overclocking, which shows little benefit.
We can't really determine where the performance difference between two and four CPU cores is coming from within our data. With AA enabled, the graphics memory is almost fully used at 750 MB. The CPU utilization goes down when overclocking, but there is no frame rate increase.
To get higher frame rates, you'll want to upgrade your graphics card and use at least a quad-core CPU, despite just two of the cores being fully used.