Is this the kind of motherboard you are looking for?
2 x Socket G34 compatible
AMD Opteron 6100 series
AMD Opteron 6200 series
AMD Opteron 6300 series
Slot Location 2 : PCI-E x16 (Gen2 X8 Link)
Slot Location 3 : PCI-E x16 (Gen2 X16 Link), Auto switch to x8 link if slot 2 is occupied
That is probably the best you are going to find.
AMD has not supported PCI-E 3.0 because the chipsets do not have enough bandwidth to support it. I see some AMD motherboards coming out with support for PCI-E 3.0 but they are only for single socket AM3+ CPUs. These days it seems AMD seems to give workstation/server products a lower priority than they used to so it may be a while before they support PCI-E 3.0 in that market.
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