What is required for SLI is a strong PSU , motherboard, and 2 PCIe x8 slots. They can be both x16 but one running in x8 mode.
However, I don't recommend SLI. The GTX 680 is already a very strong card. SLI would increase the temperatures and consume a lot more power.
But if you have a water cooling system and a don't mind the higher power consumption, then sure go ahead. But take note that not all games are compatible with SLI
There are some rumours of new GPUs from Nvidia coming up this year so that's one option. Or you could go with the GTX 780 ti with extra VRAM