The GTX 660 is due sometime in September - rumor is next week. Pricing is expected in the mid $200's. Rumor has it to be a new die - GK106. As usual, Tweaktown defies the launch date and publishes an early review. From this one datapoint, it looks like the GTX 660 will slot in between the HD 7870 and the HD 7950, game dependant. Compared to the 660 Ti, performance is 6 to 22% slower, with an average of 14% for the eight games tested at 1920x1200. Turning the knob to Max AA and/or Max AF: the 660 is 8% slower on average for the 5 games tested compared to the 660 Ti, again at 1920x1200.
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Sept. 13: Reviews are out. Looks like performance slots in between the 7850 and 7870. Closer to the 7870 except for a few ATI tuned games and ones with heavy shader use. Nice idle power improvement over the GTX 660 Ti at 7 to 8W and great two monitor power use at ~9W. Over all, a sweet GPU for 1080p gaming. Power target of 115W and TDP of 140W.

Detailed game performance at The Tech Report.

Asus DCII TOP
Tech Report
Hardware Info
Techpowerup
Hardwareluxx (in Googlish)
Guru3D

Asus DCII OC
X-bit Labs

MSI OC Twin Frozr III 3 heatpipe version
HT4U (in Googlish)
Techpowerup
KitGuru
Guru3D
Hardware Canucks
Tweaktown

MSI Hawk with TF IV 4 heatpipe version. Good news: TF IV! Bad news: New power regulation adds ~6W across the board (idle thru stress).
http://www.techpowerup.com/reviews/MSI/GTX_660_HAWK/27.html
Guru3D

Zotac
Tech Report
Hardware Info
X-Bit Labs
Techpowerup
Hardwareluxx

Gigabyte Windforce
Techpowerup
Guru3D
Hexus

EVGA
HT4U
Hardware Canucks
Guru3d

KFA2 EX OC
Guru3D
Hexus

Inno3D iChill HerculeZ
Guru3D

Not really once you factor in their old drivers and drop the irrelevant benchmarks it very much looks like it's going to be a bit below HD7870 on average. OC versions though I guess could be really close (not too much headroom for OC I suppose however). I _think_ it's going to be priced accordingly (not more than 250USD).
Release date should be Sept 12.

First review on Tweaktown speed only not power consumption or noise:
"...[The 660] ultimately sits perfectly against the HD 7870."

Intro day reviews added.

Another MSI and EVGA review added.

All these review links are great.
But, it would be even better if our own SPCR reviewed some of these.

Two more Gigabyte Windforce reviews added.

Inno3D added - Guru3D is impressed.

MSI Hawk with Twin Frozr IV added.

i love my msi twinfrozr 3 660. its freaking quiet, amd overclocks boost to 1176 nicely

Asus GeForce GTX 660 DirectCU II OC 2 GB reviewed at X-Bit Labs.

They plot the sound levels (to a degree), commenting that "[this Asus card] is in fact one of the quietest graphics cards we’ve ever tested."

added.