I built a new PC this summer. I originally ordered a Sapphire Radeon 6950 (OC edition). I was not sufficient impressed with it's performance, so I ordered another (you can get 2 for about $400 on sale.)
I have spent a lot of time trying to overclock the video cards while being fully stable and not getting too hot.
I believe I am done, or mostly done anyway...always seems like you think everything is working, then it crashes again, etc.
My overclock settings are 835 Mhz/1375 Mhz (core/memory) with stock voltage.
The CPU is a locked i7 3770, with a slight base clock boost to 103 Mhz. The mobo is an Asus P8Z77V (the full details are in the homebuilt PC thread somewhere recent.)
I had a lot of heat issues with the top card (the one who's airflow is blocked by the bottom card) in spite of a decent gap. I added a side fan, but that was only somewhat helpful (hard to tell.) That forced me to back off on the voltage boost (the hot electronics smell that often indicates burned up electronics also scared me off.)
Approximate fps in Dirt 3, with ultra preset, 8x MSAA and 1920x1080 rez is about : 80fps (average; this is dissapointing, I hoped for better)
Diablo 3 : Call it 170 to 220 fps; Seems best to just turn on vsync, and run cooler and quieter
Heaven 3.0 benchmark; 73.9 fps, score: 1861, min/max fps: 26.6, 148.8min/max fps: 26.6, 148.8
Normal tess., high shaders, 16x Anis. 8x AA a
(Extreme Tess; 49.4 fps, score: 1243, min/max fps 18.1/118.5)
Probably acceptable $400 performance. I would guess a single GTX 670, overclocked sufficiently will match or better it. Based on what I have seen the 6950x2 in my system with my overclock, will beat GTX 670 stock in some areas.