SLi allows either frame swapping, or half-screen. It really all depends on which game it is...and they need to be supported by the driver. I am sure that the driver support will expand greatly with updates, but they only about 25 or so games currently supported.
Any SLi motherboard comes with the PCB inter-connect for connecting the 2 cards. Also be aware that you
cannot connect just any 2 Nvidia pci-e cards. They must be of the same model, and bios as well. Oh, and they need to have the SLi pcb cutout. Make sure to get a robust power supply as well, and definately don't spare cash on the
psu if you plan to overclock.