GeForce GTX 770M SLI is a combination of 2 GeForce GTX 770M put together using NVIDIA's SLI technology.
Together, this combination offers 1920 Shader Processing Units, 160 TMUs and 48 ROPs. This, on a 192-bit memory interface, per card, of fast GDDR5. While the central unit runs at 811MHz and goes up to XXXMHz, in Turbo Mode, the memory clock operates at 1000MHz.
Best case scenario, this combination is expected to perform better a single GeForce GTX 780M and deliver similar performance to a desktop GeForce GTX 680 but ultimately, its performance depends on the driver support and on the 3D application.
Therefore, and paired with a powerful processor, all demanding games should be playable at the highest settings. Still, this combination might surpass the 150W barrier of TDP and therefore require more power than a single GeForce GTX 780M, though delivering better performance.