amd boards have nothing to do with sli, the manufacturer of the board is who controls that.
but there is no use for sli or crossfire in your system as the specs on newegg show the the 2 lanes are running @ x16/x4, so the second card won't be running at over half speed. a 6950 2gb would be great, but a 6950 or 560ti will also perform well
AMD do have to do with SLI, it isn't the manufacturer, it is the chipset.
As Nvidia do not develop chipsets any more and the only chipset developers that makes then SLI capable are Intel, only AM3 nForce boards are true SLI capable, or 9xx AM3+ chipsets.
you can do a software SLI, but it isn't "official".
Also, the PCIe x4 does not mean the card will be running at half the speed it should be, it just means the total bandwidth is less. The performance may be slightly lower, but you are talking maybe 5% top, a 460 doesn't saturate a PCIe x4 slot.
to explain - If you have 2 pipes, one that can have 10 litres a second and one that can have 100 litres a second and you are sending 5 litres a second down each, the one which has a higher maximum flow amount will not magic up more water, both will still be throwing 5 litres/second out of the end.
Personally, I would stick with the 460 and wait until Q1/Q2 next year for the 600/7000 series