I remember I had 2 GTX660 OCs in SLI and it was a fast ah heck. I had it in a 3770k OC rig. Biggest drawback with high res monitors is the amount of Vram and size of memory bus.
I now have 2 GTX670 FTWs and obviously it's faster.
That being said, what are your system specs ?
If you already have a GTX660 OC and can pick up a second inexpensively, might be a great way to quickly boost frame rates on a 1920x1080 monitor.
There are some threads in this forum, if you search, that specifically deal with the 660 in SLI and what a great deal they are for the price. Like all video cards, time marches on.