SLI or crossfire on card is awesome, gives nice fps on single monitor even those fps are wasted, but well let's go back to reality. having more than 1 card on a single monitor is not good. even if your using 2 lower tier compared to buying 1 better card(unless your adding a 2nd card). as you will be having multiple head ache but can give great eye candy.
I my self would go with sli/crossfire since I use multi monitor and looks awesome, but well also another down side is, your electric bill "might" burn out your bank
gtx 560ti SLi is going to perform similar to a 680.but i would suggest 680.it's a newer card.it has the following pros over 560ti Sli-
great performance even at stock speeds.
no micro-stuttering and compatibility issue.
less heat and noise.
560ti Sli cons-
compatibility issues in many games.
lotta heat and noise(in comparison to a single 680).
no upgrade in future.
micro-stuttering and driver issues.