New video card not detected on Asus M3A79-T Deluxe

I just purchased a sapphire radeon HD 6850 to replace my old radeon 4850. My motherboard is a M3A79-T Deluxe (specs here), and the power supply is a 700w ocz (here).

On boot the 6850's fan runs full speed, the mb beeps out the "no vga detected" sequence, and no video appears.

Here's what I've tried:
Unplugging power from all drives and removing all other cards, no luck.
Several of the pci-e slots, no luck.
Both of the pci-e power connectors on the psu. I even tried the 4-pin molex to 6-pin adapter included with the card, no luck.
I put the graphics card in another computer with a newer mb and 480 watt Antec psu and it boots fine.

My prime suspect is the PSU, but 700w seems like more than enough and it's on the list of certified PSUs for the graphics card.

Can anyone smarter than me point out what I am missing?

Update: I tried the Antec psu (from the rig where it worked) with the M3A79 and it still doesn't work, leaving only the M3A79 as the likely culprit. For whatever reason it appears that the motherboard is not providing enough power to the new graphics card.

I'm giving up on the motherboard. Very dissapointed.
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  1. You are correct - that PS should be enough to power the 6850. I would try leaving the old 4850 in and put the 6850 in another slot.

    Can you get to the OS? If so, do you see the 6850 in the device list? If so, use the AMD Vision Control Center->Desktop Management-Creating and Arranging Desktops to set up the desktop area, color quality, refresh rate, and rotation (Desktop Properties).

    Then switch the monitor cable to the 6850 and see if you get your desktop. If so, make it the primary display in Creating and Arranging Desktops by right-clicking on the display and selecting Make Primary. Now you can power down and remove the old 4850 and everything should work correctly.

    A 700W power supply should be enough to power both cards under minimal load for a while, but I would remove the 4850 before trying to view videos (or play a game) to make sure I didn't overstress the PS.
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