"Support for unregistered DIMMs up to PC2 8500(DDR2-1066MHz) and PC3 10600 (DDR3-1333MHz) – AM3"
Will it run memory at faster clocks? MAYBE, depending on the luck of the draw and such. All the board does is supply power. It's up to the integrated memory controller on the CPU to actually operate the DRAM.
Will you benefit from higher memory clocks? Not really.
You asked about x16. 890GX boards operate at x8/x8 speeds in crossfire. 890FX chipset will supply full bandwidth and are not out yet.