CornellEngineer,
If you expand the testing to more than 5-6 games and include more modern games, the picture is different. GTX680/HD7970 GE are 46-53% faster at 1080P and 47-65% faster at 1600P with AA than a 660:
http://www.computerbase.de/artikel/grafikkarten/2012/test-nvidia-geforce-gtx-660/5/
Let's assume 660 SLI has 80% scaling. So you'd get 180% vs. say 153% for HD7970 GE at 1080P. That's only 18-20% faster when SLI scaling works on average and a whopping 50%+ slower when SLI doesn't work (which happens for some new games until SLI/CF profiles are updated).
Now, then take a 7970 GE/GTX680 and overclock both of those too. Is GTX660 OC SLI going to give you better frames, sure probably by 20-30%, but when the scaling is poor you get $229 level of performance and in GPU demanding scenarios with MSAA/mods, GTX660 SLI will suffer from micro-stutter at similar frames. That's because you need higher framerates on dual-cards to have a similar level of smoothness vs. 1 card once you reach lower frames (i.e., 45 fps on a high-end flagship can often feel as smooth at 60 fps on dual mid-range cards).
Also, 660s have severe memory bandwidth and ROP limitations which means performance drops like a rock with MSAA and mods even in today's games, nevermind future games. For example in Skyrim + ENB mods, 7970 GE is 73% faster with 8xMSAA than a 660 is. So if you get 2x 660s, you will only match a single 7970 GE in that game.
http://www.computerbase.de/artikel/grafikkarten/2012/test-nvidia-geforce-gtx-660/23/
There are also other games such as
Alan Wake where 660 is nearly half as fast as a high-end flagship, or
Metro 2033 where a 660 is only as fast as a 7850 because of its memory bandwidth/ROP issues.
Because micro-stutter becomes more evident the more GPU demanding a game is as it pushes frame rates lower, it'll become more evident when next generation of games come on 660 SLI. This is why SLI/CF generally only work well for flagship products in reality. In theory the mid-range cards put up good CF/SLi performance but real world gameplay reveals micro-stutter.
660 SLI will beat 670 no doubt but if you must spend $460, you can find an HD7970 GE or a
GTX680 for that much as well. Personally, I'd rather take 50% faster performance almost all the time, than 20% faster performance only when SLI scaling works well. This is why GTX660 SLI doesn't make sense to me at $460. And actually, I'd say HD7950 @ 1.1ghz overclock in CF for
$560 is well worth the extra $100 over 660 SLI or if you must go NV find 660Tis on sale.
Also, because 660/660Ti cards are memory bandwidth/ROP limited, the SLI scaling is nowhere near 80% on average:
http://www.computerbase.de/artikel/grafikkarten/2012/test-nvidia-geforce-gtx-660-ti/7/
I'd rather get a 680 or 7970 GE for $450-460 vs. 660 SLI or spend $100 more for HD7950 Cross-fire and overclock them.
An overclocked 7950 = GTX680/7970 GE for hundreds less. That's $100 well spent for 7950 CF vs. 660 SLI.