Therse two PCIE16 slots, it wont run at 16x on the bottom slot but only 8x.
doesn't matter since these cards don't utilize above 8x bandwidth anyway.
There are lots of people over at www.dfi-street
who put their card in the bottom slot.....some report that their chipset runs cooler and theres been no performance loss and for a few people a few points (literally like 5 points more) in 3dmark05......which basically means squat, but hey atleast it's not a 500 point drop.
You have to enable SLI by moving the jumpers and then you should be able to put your card in the bottom slot.
For more on it go to www.dfi-street.com
and talk to some people there, or rather simply do a search before posting a question that's been answered many times over.