Adam, congrats on the new card!!
I think to answer what kind of overclock you should expect, it depends on your ASIC and also the type of workload you run.
I don't believe in 3 min run of 3dMark11 as stability testing. The program is not enough to stress the GPU. It's evident in the GPU usage and in the temperatures at load it generates. Not all games are going to be as easy to pass as 3dMark11 either. I think even Unigine Heaven 3.0 with Extreme Tessellation + 8AA and Metro 2033 with 4xMSAA, DOF + tessellation are more stressful.
Here is 24 hours at 99% GPU usage:
If you notice there is a slight undervolt on my card from MSI Afterburner vs. GPU-Z. That's common for 7970 style cards. My Asic is a low 71%. I think on 1.174-1.175V, a reasonable overclock is 1150-1185mhz. 1200-1250mhz generally requires 1.25V but could be lower if all you do is run games which almost never use up 99% of GPU for 5 hours at a time.
I find my VRMs exceed 100*C easily at 1.25V and 1220mhz clocks, and power consumption goes up almost 50W. Not worth it to me.
I use
HWinFO64 to monitor VRM temperatures.
If you'll be using this for gaming only I'd say 1185 is probably realistic with stock voltage. The amazing 1250-1300mhz overclocks on air we read about are actually very rare on these cards, and most of those require 1.3V. Power consumption skyrockets 100W over 1.175V. I think 1.175-1.2 Ghz with a slight voltage bump is probably a reasonable average for air cooled 7970s for actual real world usage and not just benchmarking.