K K0 said:
Will the motherboard automatically lower the speed or is it incompatible?
You can try, but it's not guaranteed.
Intel Sandybridge 2000 series desktop processors natively support DDR3-1333, but the quality of the memory controller means that it will usually run all JEDEC standard profiles up to and including DDR3-2133 without much of a fuss. Running DDR3-1600 on this platform is technically overclocking but it will run flawlessly well over 95% of the time.
AMD's processors are a little more finicky. There have been reports of Bulldozer CPUs having a hard time running natively supported DDR3-1866.
Motherboard quality will also have an impact as power stability becomes increasingly important as operating speeds increase.