I'm not sure tbh. I have a similar setup to you, and I've noticed that even in WoW my 6870 is at maximum usage where the CPU is only running at 70-80%. And that's only @ 3.8ghz.
You've got yours to 4.0, you shouldn't have any CPU issues.
Before you decide to upgrade CPU, open the game and check your GPU usage on it, then check your CPU usage. If your GPU is maxed out then you know its the GPU.
Ok i did a quick test, im playing WOW at 1360x768 on high preset with 8x multisampling on from the in game options. While flying around in an open area with no players my cpu goes anywhere from 70%-100% Usage and my GPU is at about 70% usage.
When i go into Orgrimmar my cpu usage is still about 70% and the Gpu is only at about 39% usage. I will keep monitoring but im still trying to find the best way to monitor while i play. Im currently using Hwinfo64