Of course it works. PCI-E is not made for graphic cards. It's for any card that supports PCI-E. 

Really? I use slot 2 only for graphic card on my 750i FTW without any problems.

You know what you should do? Give EVGA a call. They'll know the answer.

888.880.EVGA (3842)

I put a card into the slot and it appears usable, sort of.   It's an Areca ARC-1210 RAID card, and it uses a PCIe x8 connector.

When I boot the machine the BIOS doesn't bring up the card's boot ROM.  If I boot from a drive attached to the other ports the card is accessible and works in either of the other PCIe x16 slots.  But it doesn't seem to be bootable in either one of the slots. 

I've got the motherboard BIOS "Boot other device" set to Enabled.  I've got "Hard Disk Boot Priority" set with "Bootable Add-in Cards" at the #1 position.  I've tried all variations in the First, Second and Third boot device settings.  None seem to get the card ROM to boot.  Not Removable, Hard Disk, CDROM, Network nor Disabled.  No variations seem to bring up the card's boot ROM.

In the past I had a bootable card in the PCIe x1 slot and it's boot ROM would come up.  It's not present in the machine now (so it's not something conflicting here).   Right now the only two cards in the machine are an EVGA video card in the first slot and the RAID card (an Areca 1210) in one of the others (I tried both).   I've got the latest (P10) BIOS loaded on the motherboard.

I've tried the card on two other kinds of motherboards (one Asus and the other an Intel ) and they bring up the boot ROM just fine.  So the card itself is OK.

So, can the 780i BIOS boot from a card in the PCIe x16 slots? 

If not, could a BIOS patch be issued to allow this?  It's been an awful lot of work to get things setup on this board and I'm just sooooo close to completion.

Turned out the RAID card needed it's firmware updated.  Now it boots just fine.

One other question, on the 780i which of the x16 slots is faster than the others?  I realize the video card has to be in the slot closest to the CPU (often the left slot when laid flat, or topmost when vertical).  I have my video card in that slot.  But into which slot should I put the RAID card?  Does it matter?  It's a x8 card, if that matters.

I'll also point out to others that an x1 card works just fine in the x16 slots.  Bootable too.  I have a x1 jmicron esata card and it works perfectly in any of the PCIe slots.  Just an FYI.