Originally Posted by newegg.com
2 x PCI Express x16 slots, running at x16 (PCIEX16_1/PCIEX16_2) (Note 1)
2 x PCI Express x16 slot, running at x8 (PCIEX8_1/PCIEX8_2) (Note 2)
(Note 1) For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_2 slots.
(Note 2) The PCIEX8_1 and PCIEX8_2 slots share bandwidth with the PCIEX16_1 and PCIEX16_2 slots espectively. When PCIEX8_1 is populated with an expansion card, the PCIEX16_1 slot will operate at up to x8 mode; when PCIEX8_2 is populated with an expansion card, the PC IEX16_2 slot will operate at up to x8 mode.
but my best guess says to put the cards in the first and 3rd slots