I have to say it's likely your CPU creating a bottleneck
I run it on pretty much the equivalent of your GPU (7970 OC), and a stock i7 950, with 8GB 1600 HyperX RAM, and I am playing with textures, shadows and foliage maxed, with HBAO+ and AA on, Blur, DoF and Vsync off, at 1080p, and getting at least 40-50 FPS average if not more.
I HAVE however used several file tweaks to disable film grain, noise, aberration, cloud shadows, and desaturation, but I've also doubled the saturation and am running a SweetFX preset.
It may not be one of the more common brand PSUs you're using, but I doubt it's the culprit.