Asus P5N-D - Manual (Page 158)

6-6 Chapter 6: NVIDIA ® SLI™ technology support
5. From the Display Properties dialog
box, select the Settings tab then
click Advanced.
6. Select the NVIDIA GeForce tab.
7. Click the slider to display the
following screen, then select the SLI
multi-GPU item.
8. Click the Enable SLI multi-GPU check box.
9. Click OK when done.
Slider