Try to bridge the pci-e slot to force VGA detection, even if its a x16 slot, i noticed some problems booting up 58xx and 68xx on some mbs, especially older ones, bridge it to force detection fixed it in most cases.
No matter if its a x16 slot, just bridge those pins.
to "TheGer"; yeah, you are right in that, my main rig is based on a M2N4 SLI mb, with 4 Sapphire xtreme HD5850, 3 of those boot up just fine in any of the x16 slots, but one of them negates to boot up on the pci-e x16 slots for some reason!, bridging the x16 slot to force vga detection fixed the problem. And i had to bridge the x1 slots for all cards.
is no one reading anything? I already posted this solution, but the OP is not responding to the thread, just posting in other ones