You probably won't have to do anything. After you install both your cards and update the drivers for them, right click on screen, go to properties, settings, advanced, click on nvidia 7800 or whatever...up top.... the selection for sli should be there somewhere.on the screen in front of you. If it doesn't show on first screen, open a couple of things using the window that pops up to the left. If you have to go to the left popup screen, select whatever is at the top first, I think it's there.........
I have one of these boards. In the bios there are 2 things relating to sli. They look very similar, and I didn't touch them at all while building machine.