Maxing Out MOBO/CPU

I recently built a PC for gaming and to this point it works pretty good for my gaming purposes. I've mainly been playing BF3, TERA and Skyrim and Witcher 2, with Guild Wars 2 and several other upcoming titles waiting in the wings.

My build is as follows:

CPU: AMD Phenom II X4 965 Black Edition
MOBO: AsRock 970 Extreme3
RAM: Corsair Vengeance 8GB DDR3 1600 PC3 12800
PSU: Seasonic X650 Gold
GPU: Sapphire Radeon 6850-2GB
SSD: Corsair Force Series 120GB
HDD: Western Digital 500 GB

Although this is a relatively new build, I was wondering, with the MOBO, what CPU would be the max that I could use (it supports AM3+), and if it would be worth upgrading it or by overclocking my current one will I get close to the performance?

Also, I have posted a question about the GPU (crossfire with another) and if I were to upgrade the CPU, should I upgrade the GPU and better RAM as well?