Would it be an upgrade or downgrade if i got 2x SLI GTX 660's and got rid of my gtx 670?
downgrade
the 660 has a 192 bit memory bus which will negate any advantage of SLI
The 192 Bit bus is enough, it will definitley outperform a 670 and 680
here are some links showing said benefits of SLI
http://www.guru3d.com/articles_pages/kfa2_geforce_gtx_660_ex_oc_review,24.html
and here's the 4GB 680 it beats
http://www.guru3d.com/articles_pages/evga_geforce_gtx_680_classified_with_evbot_review,19.html
It Uses BF3 because its one of the most demanding games, but the scaliling usually applies the same to other games, you can see 25+ % more performance from SLI'd GTX 660's compared to a 680 4GB
only at resolutions of 1920x1080 or less, after that and you start to see extreme bottlenecking due to the 192 bit bus
also BF3 is not that demanding
If you take a look at Crysis 2 (with High-res textures) at 2560 x 1600 you would see that the 4Gb 680 gets 37Fps and the SLI 660's get 51, still around 20% better, honestly unless running something like surround the memory bus isnt going to play such a crucial role, and the GTX 660's are actually very good Cards for SLI as they scale fairly well.
I agree, the 660's in SLI would definitely be an improvement. I actually have a 660 right now and am planning to add a second one later. Some people say that SLI is buggy or that you're cards will overheat, but honestly, Nvidia does a great job with SLI scaling as far as increasing FPS and reducing stuttering. This article helps to show that.
http://www.tomshardware.com/reviews/graphics-card-benchmarking-frame-rate,3466.html
As far as heating goes, the EVGA GTX 660 that I have never gets above 61 degrees, and that's using the default fan curve. It also uses the the blower style fan, so if you have two cards, card A won't blow hot air onto card B, so there shouldn't be much of a temperature increase at all. Just remember, you will need extra power for two cards, but 650 - 750W should be plenty. Also note that you've maxed out your upgrades with 2 660s because they don't support 3 or 4 way SLI.
Overall though, if you want to see a good increase in performance, go for those 2 660s!
LOL at this point it'd probably be a dumb decision. Just keep the 670 as it's already a badass card and wait for the 700 or whatever the next series is. Or you could just get another 670..
I agree!
Instead of messing around selling your 670, buy another and just SLI your 670. Will be much less hassle even though slightly more costy but also a better performance