PC3 12800 is DDR3 1600
PC - theoretical bandwidth. More about RAMs: http://en.wikipedia.org/wiki/DDR3_SDRAM
Faster is not always better. Depending on the motherboard, try using the configurator for finding compatible kits: http://www.mushkin.com/Memory/Configurator.aspx
And since the memory controller is integrated into the CPU, it also matters when it comes to RAMs specs.
Using RAMs speeds above the factory/recommended values implies higher temperatures, shorter components' life a.s.o.