gib6717 :
I hit best solution on accident! (but maybe it is the best!)
Is the sli option the best for now? I've read so many bad things about it that I'm not sure!
I unselected the solution since you hit it by accident. Feel free to select this again or some other answer.
SLI will increase the frame rate compared to a single card with the same settings.
This isn't the same as saying you can use higher settings with SLI because of limitations like video RAM.
SLI can be good in some situations. Probably the best application is people aiming for high frame rates on 120Hz monitors. For what you are doing, it isn't particularly helpful.
On the comment from gridironcj, the GTX 660 is the same generation of technology from Nvidia as every other available card except for the GTX 750 Ti (which is slower, but uses a new architecture). A single GTX 780 will be perfect for what you want, a pair or trio of them would be a huge waste of money.
A single GTX 660 will give you playable frame rates at 1440p with some reduced settings. GTX 660s in SLI will allow the same settings with higher frame rates (probably hitting at maximum 60 FPS that can be displayed by your monitor). To use higher settings you will need to look at a faster card with more video RAM instead like the AMD Radeon R9 280X, 290 or 290X or the Nvidia GTX 780 or 780 Ti.