Asus P5N-D - Manual (Page 155)

ASUS P5N-D 6-3
4. Insert the second graphics card into the other slot (PCIEX16_2). Make sure
that the card is properly seated on the slot.
If required, connect an auxiliary power source to the PCI Express graphics
cards.
3. Insert one graphics card into one of the blue slots (PCIEX16_1). Make sure
that the card is properly seated on the slot.