The RAM will improve the everyday performance more than the CPU. The E7500 has a higher base clock speed than the Q6600. But games that can use more than two cores will benefit from the additional CPU resources. There is no simple answer on "how much increase?" because it will depend on the software you are running.
I would recommend doing the RAM increase and even an SSD swap if you can. See how that works, then swap the CPU at a later time. You will be able to see which things impact your performance most.