Fun with clock speeds
Although our XFX 7950 Black Edition card came out of the box at 900MHz, 100MHz above AMD's baseline clock frequency, it still had plenty of overclocking headroom waiting to be exploited. We were able to take it up to 1025MHz using AMD's Overdrive control panel, simply by sliding the GPU core speed slider, at the default voltage of 1031 mV. (Ok, so we also raised the PowerTune TDP cap by the maximum 20% allowed in the control panel, to avoid any power-based frequency capping.) That's pretty darned good by itself.

To go further, we fired up MSI's Afterburner utility, which allows for voltage tweaking. After some experimentation, we got the core clock up to 1175MHz at 1162 mV, with GPU temperatures around 89-90° C. Higher clock speeds produced visual artifacts, even at higher voltages. Also, when pushing to higher voltages, we seemed to be surpassing the limits of the card's cooler; temperatures quickly crept up to around 96° C, which is a bit uncomfortable. We were also able to take the card's memory clock quite a bit higher. After some experimentation, we settled on a memory frequency of 1575MHz, well above the XFX card's default of 1375MHz

Bottom line: that is a frickin' lot of clock speed headroom for a GPU.

While we were messing with clock speeds, I figured we'd take this opportuntity to look at the performance of the 7950 and 7970 at their non-Black Edition clock speeds, as well.

When overclocked to 1175/1575MHz, the 7950 turns out to be even faster than the hot-clocked XFX 7970. Still, the difference between the various Tahiti-based products only adds up to a handful of frames per second, and all of them achieve a higher average than the GTX 580. When we turn to the 99th percentile frame times, the gaps between the solutions grow even smaller. In this case, frame latencies are probably limited primarily by other factors, such as CPU performance or driver execution speed.