I believe there is one AM3+ board with PCIe gen3, apparently the CPU's are capable of supporting it, its just not supported in the mobo chipset so if the board manufacturer is willing it can be added in.
To the OP, in terms of graphics cards it doesnt matter. The highest end cards are nowhere near saturating Gen2 8x, and every motherboard is guaranteed a Gen2 16x at least. Your fine, PCIe Gen3 is more of a future proofing thing for PCIe based storage solutions and networking.