AMD's ZeroCore power feature gives the Radeons an advantage when the display goes into power-save mode, as you can see. The Radeons drop into a low-power state, spin down their fans, and shave 15W or so off of total system power draw. Without this feature, the GeForces can't match them.
When running a game—Skyrim, in this case—the GTX 660 Ti cards draw quite a bit less power than anything else in the field, including the competing Radeons. The Kepler architecture has proven to be very power efficient, and that goodness extends to its latest derivative.
Noise levels and GPU temperatures
With their fans spun down in ZeroCore mode, the Radeons approach our system and test environment's noise floor. Then again, the Zotac GTX 660 Ti card is right there with them, though its fans are spinning.
To me, the biggest story of the results above is the effectiveness of MSI's custom dual-fan coolers, which capture the top three spots for lowest noise levels under load. MSI's GTX 660 Ti card is the quietest of the bunch, which makes sense given its more modest power draw; it has less heat to dissipate than the 7870 or 7950.
Beyond that, the Zotac GTX 660 Ti AMP! card deserves some praise for combining the lowest noise levels at idle with a mid-pack performance under load and a modest peak temperature of 67° C. I'd have preferred fan tuning that's biased a little more toward quiet than cool, but that stubby little cooler does look to be pretty effective.
Meanwhile, PNY's decision to use the stock Nvidia cooler looks unfortunate, since it combines high temperatures with relatively high noise levels, despite the GTX 660 Ti's modest power draw (and thus modest heat generation). This cooler still isn't terribly loud, like the reference 7950's with boost is, but the custom coolers simply outperform it.