Anandtech have a review of the GTX 660 and GTX 650,
The NVIDIA GeForce GTX 660 Review: GK106 Fills Out The Kepler Family by Ryan Smith.

For here I would expect the performance of the GTX660 to be ~75% of the GTX660Ti. In terms of performance per initial outlay ($/£/Euro...) that's almost spot on the same as a GTX660Ti (~76% of the cost). Running costs should also be less, possibly quite reasonable in comparison (say around 90% as efficient in terms of performance/watt), though it's unlikely that any will quite match the GTX660Ti. The range of GTX660 cards will probably see performances of some models reach 85% of a reference GTX660Ti.

Voltages seem to be fixed on the GTX660's, but with some manufacturer variety; 1.175v, 1.162v.

As with the GT640 the GTX650 has no boost clock, so your stuck with whatever ships, be that reference or some manufacturer specific/bespoke setting. The reference model should be around 20% faster than a GT640 for here.
With only 384 shaders your not going to get much performance out of this, but in terms of performance per outlay your talking around 86% of a GTX660Ti, which is quite reasonable for a low end card.

To me it appears that there is still a sizable performance gap between the GTX660 and the GTX650. Hopefully a GTX650Ti will arrive to complete the range; an on the shelf GK106 with 4 SM's would be sweet, rather than some dodgy hobbled oem version with 3 SM's and DDR3 ;P
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So far all the cards I have seen (both GTX660 and GTX650) require one 6-pin PCIE power connector.
5years ago some motherboards with 4 PCIE slots that could support 4 x 75W.
It seems odd that they now feel the need to have a 6-pin power connector on a 64W card (GTX650), especially when you can't up the Voltage and when the GTX660 presumes it can draw up to 75W from the slot to support it's 140W TDP.
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I am looking at some GTX660's as replacements for GTX560Ti's. Not intended for use here. I already have a couple of factory OC'ed GTX670's that I use specifically for GPUgrid.

They should give a bit of a performance boost while reducing power consumption.
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