From what I remember the eVGA 680i boards have SPDIF out on the back panel, and just beside the front panel audio connector on the motherboard (bottom left of the board) there should be a set of 5 pins that are related to SPDIF out. The real trick is that you only need 2 of them. Get them wrong, and you could toast something as pin 1 is for power.
Edit: A quick search of eVGAs site turned up
THIS thread. There's a picture in there for a another board, but it's the same for the 680i. The key is to hook up pin 2 (SPDIF out) to pin 1 on the video card (could be shown as a triangle, a + sign, or simply labeled with a 1), and pin 5 (ground) to the other pin on the video card.