Asus Motherboard A8N-SLI SE - Manual (Page 147)

ASUS A8N-SLI SE 6-3
4. Insert the second graphics card into the black slot labeled 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 the blue slot labeled PCIEX16_1. Make
sure that the card is properly seated on the slot.