http://cdn.wccftech.com/wp-content/uploads/2013/06/Battlefield-4-1920x1080-4x-MSAA.jpg
660 sli is on par with gtx 590 and even better in some cases, considering that with every setting MAXED OUT in an UNoptimised ALPHA version without proper drivers gtx 590 sees average of 48 FPS you can expect to see minimum 60 FPS in released retail version with proper drivers on your setup.
Bf2-Bf3 6 years of time
bf3-bf4 2.5 years of time
Not really a big jump there actually.
http://www.xbitlabs.com/images/video/geforce-gtx-580-sli/t1920.png
Here is a sli gtx 580 vs gtx 590. Note that gtx 580SLI is very similar to gtx 660 sli and in this comparison gtx 580 sli wins against gtx 590 by an average of 16% performance. So gtx 660 sli is at worst 10% better than a gtx 590, puts it right behind gtx 780 although gtx 780 clearly wins.